[RFC PATCH v3 0/8] Sparse checkout

@@ -162,7 +162,8 @@ int run_diff_files(struct rev_info *revs, unsigned int option)
 		if (ce_uptodate(ce))
 			continue;
 
-		changed = check_removed(ce, &st);
+		/* If CE_VALID is set, don't look at workdir for file removal */
+		changed = (ce->ce_flags & CE_VALID) ? 0 : check_removed(ce, &st);
 		if (changed) {
 			if (changed < 0) {
 				perror(ce->name);

@@ -337,6 +338,8 @@ static void do_oneway_diff(struct unpack_trees_options *o,
 	struct rev_info *revs = o->unpack_data;
 	int match_missing, cached;
 
+	/* if the entry is not checked out, don't examine work tree */
+	cached = o->index_only || (idx && (idx->ce_flags & CE_VALID));
 	/*
 	 * Backward compatibility wart - "diff-index -m" does
 	 * not mean "do not ignore merges", but "match_missing".

@@ -344,7 +347,6 @@ static void do_oneway_diff(struct unpack_trees_options *o,
 	 * But with the revision flag parsing, that's found in
 	 * "!revs->ignore_merges".
 	 */
-	cached = o->index_only;
 	match_missing = !revs->ignore_merges;
 
 	if (cached && idx && ce_stage(idx)) {

@@ -0,0 +1,31 @@
+#!/bin/sh
+
+test_description='diff with assume-unchanged entries'
+
+. ./test-lib.sh
+
+# external diff has been tested in t4020-diff-external.sh
+
+test_expect_success 'setup' '
+	echo zero > zero &&
+	git add zero &&
+	git commit -m zero &&
+	echo one > one &&
+	echo two > two &&
+	git add one two &&
+	git commit -m onetwo &&
+	git update-index --assume-unchanged one &&
+	echo borked >> one &&
+	test "$(git ls-files -v one)" = "h one"
+'
+
+test_expect_success 'diff-index does not examine assume-unchanged entries' '
+	git diff-index HEAD^ -- one | grep -q 5626abf0f72e58d7a153368ba57db4c673c0e171
+'
+
+test_expect_success 'diff-files does not examine assume-unchanged entries' '
+	rm one &&
+	test -z "$(git diff-files -- one)"
+'
+
+test_done

@@ -229,10 +229,9 @@ static int add_excludes_from_file_1(const char *fname,
 
 	if (buf_p)
 		*buf_p = buf;
-	buf[size++] = '\n';
 	entry = buf;
-	for (i = 0; i < size; i++) {
-		if (buf[i] == '\n') {
+	for (i = 0; i <= size; i++) {
+		if (i == size || buf[i] == '\n') {
 			if (entry != buf + i && entry[0] != '#') {
 				buf[i - (i && buf[i-1] == '\r')] = 0;
 				add_exclude(entry, base, baselen, which);

@@ -350,6 +350,12 @@ static int excluded_1(const char *pathname,
 			int to_exclude = x->to_exclude;
 
 			if (x->flags & EXC_FLAG_MUSTBEDIR) {
+				if (!dtype) {
+					if (!prefixcmp(pathname, exclude))
+						return to_exclude;
+					else
+						continue;
+				}
 				if (*dtype == DT_UNKNOWN)
 					*dtype = get_dtype(NULL, pathname, pathlen);
 				if (*dtype != DT_DIR)

@@ -58,6 +58,9 @@ The result of the enumeration is left in these fields::
 Calling sequence
 ----------------
 
+* Ensure the_index is populated as it may have CE_VALID entries that
+  affect directory listing.
+
 * Prepare `struct dir_struct dir` and clear it with `memset(&dir, 0,
   sizeof(dir))`.
 

@@ -71,8 +71,11 @@ int cmd_clean(int argc, const char **argv, const char *prefix)
 
 	dir.flags |= DIR_SHOW_OTHER_DIRECTORIES;
 
-	if (!ignored)
+	if (!ignored) {
+		if (read_cache() < 0)
+			die("index file corrupt");
 		setup_standard_excludes(&dir);
+	}
 
 	pathspec = get_pathspec(prefix, argv);
 	read_cache();

@@ -481,6 +481,9 @@ int cmd_ls_files(int argc, const char **argv, const char *prefix)
 		prefix_offset = strlen(prefix);
 	git_config(git_default_config, NULL);
 
+	if (read_cache() < 0)
+		die("index file corrupt");
+
 	argc = parse_options(argc, argv, prefix, builtin_ls_files_options,
 			ls_files_usage, 0);
 	if (show_tag || show_valid_bit) {

@@ -508,7 +511,6 @@ int cmd_ls_files(int argc, const char **argv, const char *prefix)
 	pathspec = get_pathspec(prefix, argv);
 
 	/* be nice with submodule paths ending in a slash */
-	read_cache();
 	if (pathspec)
 		strip_trailing_slash_from_submodules();
 

@@ -200,11 +200,36 @@ void add_exclude(const char *string, const char *base,
 	which->excludes[which->nr++] = x;
 }
 
+static void *read_assume_unchanged_from_index(const char *path, size_t *size)
+{
+	int pos, len;
+	unsigned long sz;
+	enum object_type type;
+	void *data;
+	struct index_state *istate = &the_index;
+
+	len = strlen(path);
+	pos = index_name_pos(istate, path, len);
+	if (pos < 0)
+		return NULL;
+	/* only applies to CE_VALID entries */
+	if (!(istate->cache[pos]->ce_flags & CE_VALID))
+		return NULL;
+	data = read_sha1_file(istate->cache[pos]->sha1, &type, &sz);
+	if (!data || type != OBJ_BLOB) {
+		free(data);
+		return NULL;
+	}
+	*size = xsize_t(sz);
+	return data;
+}
+
 static int add_excludes_from_file_1(const char *fname,
 				    const char *base,
 				    int baselen,
 				    char **buf_p,
-				    struct exclude_list *which)
+				    struct exclude_list *which,
+				    int check_index)
 {
 	struct stat st;
 	int fd, i;

@@ -212,20 +237,26 @@ static int add_excludes_from_file_1(const char *fname,
 	char *buf, *entry;
 
 	fd = open(fname, O_RDONLY);
-	if (fd < 0 || fstat(fd, &st) < 0)
-		goto err;
-	size = xsize_t(st.st_size);
-	if (size == 0) {
-		close(fd);
-		return 0;
+	if (fd < 0 || fstat(fd, &st) < 0) {
+		if (0 <= fd)
+			close(fd);
+		if (!check_index ||
+		    (buf = read_assume_unchanged_from_index(fname, &size)) == NULL)
+			return -1;
 	}
-	buf = xmalloc(size+1);
-	if (read_in_full(fd, buf, size) != size)
-	{
-		free(buf);
-		goto err;
+	else {
+		size = xsize_t(st.st_size);
+		if (size == 0) {
+			close(fd);
+			return 0;
+		}
+		buf = xmalloc(size);
+		if (read_in_full(fd, buf, size) != size) {
+			close(fd);
+			return -1;
+		}
+		close(fd);
 	}
-	close(fd);
 
 	if (buf_p)
 		*buf_p = buf;

@@ -240,17 +271,12 @@ static int add_excludes_from_file_1(const char *fname,
 		}
 	}
 	return 0;
-
- err:
-	if (0 <= fd)
-		close(fd);
-	return -1;
 }
 
 void add_excludes_from_file(struct dir_struct *dir, const char *fname)
 {
 	if (add_excludes_from_file_1(fname, "", 0, NULL,
-				     &dir->exclude_list[EXC_FILE]) < 0)
+				     &dir->exclude_list[EXC_FILE], 0) < 0)
 		die("cannot use %s as an exclude file", fname);
 }
 

@@ -301,7 +327,7 @@ static void prep_exclude(struct dir_struct *dir, const char *base, int baselen)
 		strcpy(dir->basebuf + stk->baselen, dir->exclude_per_dir);
 		add_excludes_from_file_1(dir->basebuf,
 					 dir->basebuf, stk->baselen,
-					 &stk->filebuf, el);
+					 &stk->filebuf, el, 1);
 		dir->exclude_stack = stk;
 		current = stk->baselen;
 	}

@@ -64,6 +64,8 @@ two/*.4
 echo '!*.2
 !*.8' >one/two/.gitignore
 
+allignores='.gitignore one/.gitignore one/two/.gitignore'
+
 test_expect_success \
     'git ls-files --others with various exclude options.' \
     'git ls-files --others \

@@ -85,6 +87,26 @@ test_expect_success \
        >output &&
      test_cmp expect output'
 
+test_expect_success 'setup sparse gitignore' '
+	git add $allignores &&
+	git update-index --assume-unchanged $allignores &&
+	rm $allignores
+'
+
+test_expect_success \
+    'git ls-files --others with various exclude options.' \
+    'git ls-files --others \
+       --exclude=\*.6 \
+       --exclude-per-directory=.gitignore \
+       --exclude-from=.git/ignore \
+       >output &&
+     test_cmp expect output'
+
+test_expect_success 'restore gitignore' '
+	git checkout $allignores &&
+	rm .git/index
+'
+
 cat > excludes-file <<\EOF
 *.[1-8]
 e*

@@ -22,6 +22,25 @@ test_expect_success 'setup' '
 
 '
 
+test_expect_success 'git clean with assume-unchanged .gitignore' '
+	git update-index --assume-unchanged .gitignore &&
+	rm .gitignore &&
+	mkdir -p build docs &&
+	touch a.out src/part3.c docs/manual.txt obj.o build/lib.so &&
+	git clean &&
+	test -f Makefile &&
+	test -f README &&
+	test -f src/part1.c &&
+	test -f src/part2.c &&
+	test ! -f a.out &&
+	test ! -f src/part3.c &&
+	test -f docs/manual.txt &&
+	test -f obj.o &&
+	test -f build/lib.so &&
+	git update-index --no-assume-unchanged .gitignore &&
+	git checkout .gitignore
+'
+
 test_expect_success 'git clean' '
 
 	mkdir -p build docs &&

@@ -224,12 +224,12 @@ static void *read_assume_unchanged_from_index(const char *path, size_t *size)
 	return data;
 }
 
-static int add_excludes_from_file_1(const char *fname,
-				    const char *base,
-				    int baselen,
-				    char **buf_p,
-				    struct exclude_list *which,
-				    int check_index)
+int add_excludes_from_file_to_list(const char *fname,
+				   const char *base,
+				   int baselen,
+				   char **buf_p,
+				   struct exclude_list *which,
+				   int check_index)
 {
 	struct stat st;
 	int fd, i;

@@ -275,8 +275,8 @@ static int add_excludes_from_file_1(const char *fname,
 
 void add_excludes_from_file(struct dir_struct *dir, const char *fname)
 {
-	if (add_excludes_from_file_1(fname, "", 0, NULL,
-				     &dir->exclude_list[EXC_FILE], 0) < 0)
+	if (add_excludes_from_file_to_list(fname, "", 0, NULL,
+					   &dir->exclude_list[EXC_FILE], 0) < 0)
 		die("cannot use %s as an exclude file", fname);
 }
 

@@ -325,9 +325,9 @@ static void prep_exclude(struct dir_struct *dir, const char *base, int baselen)
 		memcpy(dir->basebuf + current, base + current,
 		       stk->baselen - current);
 		strcpy(dir->basebuf + stk->baselen, dir->exclude_per_dir);
-		add_excludes_from_file_1(dir->basebuf,
-					 dir->basebuf, stk->baselen,
-					 &stk->filebuf, el, 1);
+		add_excludes_from_file_to_list(dir->basebuf,
+					       dir->basebuf, stk->baselen,
+					       &stk->filebuf, el, 1);
 		dir->exclude_stack = stk;
 		current = stk->baselen;
 	}

@@ -337,9 +337,9 @@ static void prep_exclude(struct dir_struct *dir, const char *base, int baselen)
 /* Scan the list and let the last match determine the fate.
  * Return 1 for exclude, 0 for include and -1 for undecided.
  */
-static int excluded_1(const char *pathname,
-		      int pathlen, const char *basename, int *dtype,
-		      struct exclude_list *el)
+int excluded_from_list(const char *pathname,
+		       int pathlen, const char *basename, int *dtype,
+		       struct exclude_list *el)
 {
 	int i;
 

@@ -413,8 +413,8 @@ int excluded(struct dir_struct *dir, const char *pathname, int *dtype_p)
 
 	prep_exclude(dir, pathname, basename-pathname);
 	for (st = EXC_CMDL; st <= EXC_FILE; st++) {
-		switch (excluded_1(pathname, pathlen, basename,
-				   dtype_p, &dir->exclude_list[st])) {
+		switch (excluded_from_list(pathname, pathlen, basename,
+					   dtype_p, &dir->exclude_list[st])) {
 		case 0:
 			return 0;
 		case 1:

@@ -69,7 +69,11 @@ extern int match_pathspec(const char **pathspec, const char *name, int namelen,
 extern int fill_directory(struct dir_struct *dir, const char **pathspec);
 extern int read_directory(struct dir_struct *, const char *path, int len, const char **pathspec);
 
+extern int excluded_from_list(const char *pathname, int pathlen, const char *basename,
+			      int *dtype, struct exclude_list *el);
 extern int excluded(struct dir_struct *, const char *, int *);
+extern int add_excludes_from_file_to_list(const char *fname, const char *base, int baselen,
+					  char **buf_p, struct exclude_list *which, int check_index);
 extern void add_excludes_from_file(struct dir_struct *, const char *fname);
 extern void add_exclude(const char *string, const char *base,
 			int baselen, struct exclude_list *which);

@@ -31,7 +31,7 @@ static int list_tree(unsigned char *sha1)
 }
 
 static const char * const read_tree_usage[] = {
-	"git read-tree [[-m [--trivial] [--aggressive] | --reset | --prefix=<prefix>] [-u [--exclude-per-directory=<gitignore>] | -i]]  [--index-output=<file>] <tree-ish1> [<tree-ish2> [<tree-ish3>]]",
+	"git read-tree [[-m [--trivial] [--aggressive] | --reset | --prefix=<prefix>] [-u [--exclude-per-directory=<gitignore>] | -i]] [--sparse] [--index-output=<file>] <tree-ish1> [<tree-ish2> [<tree-ish3>]]",
 	NULL
 };
 

@@ -98,6 +98,8 @@ int cmd_read_tree(int argc, const char **argv, const char *unused_prefix)
 		  PARSE_OPT_NONEG, exclude_per_directory_cb },
 		OPT_SET_INT('i', NULL, &opts.index_only,
 			    "don't check the working tree after merging", 1),
+		OPT_SET_INT(0, "sparse", &opts.apply_sparse,
+			    "apply sparse checkout filter", 1),
 		OPT_END()
 	};
 

@@ -177,6 +177,9 @@ struct cache_entry {
 #define CE_HASHED    (0x100000)
 #define CE_UNHASHED  (0x200000)
 
+/* Only remove in work directory, not index */
+#define CE_WT_REMOVE (0x400000)
+
 /*
  * Extended on-disk flags
  */

@@ -0,0 +1,47 @@
+#!/bin/sh
+
+test_description='sparse checkout tests'
+
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	test_commit one &&
+	mkdir two &&
+	test_commit two two/two.t two.t
+'
+
+test_expect_success 'read-tree without .git/info/sparse' '
+	git read-tree --sparse -m -u HEAD &&
+	test -f one.t &&
+	test -f two/two.t
+'
+
+test_expect_success 'read-tree with empty .git/info/sparse' '
+	echo > .git/info/sparse &&
+	git read-tree --sparse -m -u HEAD &&
+	test -f one.t &&
+	test -f two/two.t
+'
+
+test_expect_success 'read-tree --sparse' '
+	echo "one.t" > .git/info/sparse &&
+	git read-tree --sparse -m -u HEAD &&
+	test ! -f one.t &&
+	test -f two/two.t
+'
+
+test_expect_success 'read-tree --sparse foo where foo is "directory"' '
+	echo "two" > .git/info/sparse &&
+	git read-tree --sparse -m -u HEAD &&
+	test -f one.t &&
+	test -f two/two.t
+'
+
+test_expect_success 'read-tree --sparse foo/' '
+	echo "two/" > .git/info/sparse &&
+	git read-tree --sparse -m -u HEAD &&
+	test -f one.t &&
+	test ! -f two/two.t
+'
+
+test_done

@@ -32,6 +32,12 @@ static struct unpack_trees_error_msgs unpack_plumbing_errors = {
 
 	/* bind_overlap */
 	"Entry '%s' overlaps with '%s'.  Cannot bind.",
+
+	/* sparse_not_uptodate_file */
+	"Entry '%s' not uptodate. Cannot update sparse checkout.",
+
+	/* would_lose_orphaned */
+	"Working tree file '%s' would be %s by sparse checkout update.",
 };
 
 #define ERRORMSG(o,fld) \

@@ -78,7 +84,7 @@ static int check_updates(struct unpack_trees_options *o)
 	if (o->update && o->verbose_update) {
 		for (total = cnt = 0; cnt < index->cache_nr; cnt++) {
 			struct cache_entry *ce = index->cache[cnt];
-			if (ce->ce_flags & (CE_UPDATE | CE_REMOVE))
+			if (ce->ce_flags & (CE_UPDATE | CE_REMOVE | CE_WT_REMOVE))
 				total++;
 		}
 

@@ -92,6 +98,13 @@ static int check_updates(struct unpack_trees_options *o)
 	for (i = 0; i < index->cache_nr; i++) {
 		struct cache_entry *ce = index->cache[i];
 
+		if (ce->ce_flags & CE_WT_REMOVE) {
+			display_progress(progress, ++cnt);
+			if (o->update)
+				unlink_entry(ce);
+			continue;
+		}
+
 		if (ce->ce_flags & CE_REMOVE) {
 			display_progress(progress, ++cnt);
 			if (o->update)

@@ -118,6 +131,74 @@ static int check_updates(struct unpack_trees_options *o)
 	return errs != 0;
 }
 
+static int verify_uptodate_sparse(struct cache_entry *ce, struct unpack_trees_options *o);
+static int verify_absent_sparse(struct cache_entry *ce, const char *action, struct unpack_trees_options *o);
+static int apply_sparse_checkout(struct unpack_trees_options *o)
+{
+	struct index_state *index = &o->result;
+	struct exclude_list el;
+	int i, ret = 0;
+
+	memset(&el, 0, sizeof(el));
+	if (add_excludes_from_file_to_list(git_path("info/sparse"), "", 0, NULL, &el, 0) < 0)
+		return 0;
+
+	for (i = 0; i < index->cache_nr; i++) {
+		struct cache_entry *ce = index->cache[i];
+		const char *basename;
+		int was_valid = ce->ce_flags & CE_VALID;
+
+		if (ce_stage(ce))
+			continue;
+
+		basename = strrchr(ce->name, '/');
+		basename = basename ? basename+1 : ce->name;
+		if (excluded_from_list(ce->name, ce_namelen(ce), basename, NULL, &el) > 0)
+			ce->ce_flags |= CE_VALID;
+		else
+			ce->ce_flags &= ~CE_VALID;
+
+		/*
+		 * We only care about files getting into the checkout area
+		 * If merge strategies want to remove some, go ahead
+		 */
+		if (ce->ce_flags & CE_REMOVE)
+			continue;
+
+		if (!was_valid && (ce->ce_flags & CE_VALID)) {
+			/*
+			 * If CE_UPDATE is set, verify_uptodate() must be called already
+			 * also stat info may have lost after merged_entry() so calling
+			 * verify_uptodate() again may fail
+			 */
+			if (!(ce->ce_flags & CE_UPDATE) && verify_uptodate_sparse(ce, o)) {
+				ret = -1;
+				break;
+			}
+			ce->ce_flags |= CE_WT_REMOVE;
+		}
+		if (was_valid && !(ce->ce_flags & CE_VALID)) {
+			if (verify_absent_sparse(ce, "overwritten", o)) {
+				ret = -1;
+				break;
+			}
+			ce->ce_flags |= CE_UPDATE;
+		}
+
+		/* merge strategies may set CE_UPDATE outside checkout area */
+		if (ce->ce_flags & CE_VALID)
+			ce->ce_flags &= ~CE_UPDATE;
+
+	}
+
+	for (i = 0;i < el.nr;i++)
+		free(el.excludes[i]);
+	if (el.excludes)
+		free(el.excludes);
+
+	return ret;
+}
+
 static inline int call_unpack_fn(struct cache_entry **src, struct unpack_trees_options *o)
 {
 	int ret = o->fn(src, o);

@@ -416,6 +497,9 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 	if (o->trivial_merges_only && o->nontrivial_merge)
 		return unpack_failed(o, "Merge requires file-level merging");
 
+	if (o->apply_sparse && apply_sparse_checkout(o))
+		return unpack_failed(o, NULL);
+
 	o->src_index = NULL;
 	ret = check_updates(o) ? (-2) : 0;
 	if (o->dst_index)

@@ -481,6 +565,12 @@ static int verify_uptodate(struct cache_entry *ce,
 	return verify_uptodate_1(ce, o, ERRORMSG(o, not_uptodate_file));
 }
 
+static int verify_uptodate_sparse(struct cache_entry *ce,
+				  struct unpack_trees_options *o)
+{
+	return verify_uptodate_1(ce, o, ERRORMSG(o, sparse_not_uptodate_file));
+}
+
 static void invalidate_ce_path(struct cache_entry *ce, struct unpack_trees_options *o)
 {
 	if (ce)

@@ -674,6 +764,12 @@ static int verify_absent(struct cache_entry *ce, const char *action,
 	return verify_absent_1(ce, action, o, ERRORMSG(o, would_lose_untracked));
 }
 
+static int verify_absent_sparse(struct cache_entry *ce, const char *action,
+			 struct unpack_trees_options *o)
+{
+	return verify_absent_1(ce, action, o, ERRORMSG(o, would_lose_orphaned));
+}
+
 static int merged_entry(struct cache_entry *merge, struct cache_entry *old,
 		struct unpack_trees_options *o)
 {

@@ -14,6 +14,8 @@ struct unpack_trees_error_msgs {
 	const char *not_uptodate_dir;
 	const char *would_lose_untracked;
 	const char *bind_overlap;
+	const char *sparse_not_uptodate_file;
+	const char *would_lose_orphaned;
 };
 
 struct unpack_trees_options {

@@ -28,6 +30,7 @@ struct unpack_trees_options {
 		     skip_unmerged,
 		     initial_checkout,
 		     diff_index_cached,
+		     apply_sparse,
 		     gently;
 	const char *prefix;
 	int pos;

@@ -30,6 +30,7 @@ struct checkout_opts {
 	int force;
 	int writeout_stage;
 	int writeout_error;
+	int apply_sparse;
 
 	const char *new_branch;
 	int new_branch_log;

@@ -402,6 +403,7 @@ static int merge_working_tree(struct checkout_opts *opts,
 		topts.dir = xcalloc(1, sizeof(*topts.dir));
 		topts.dir->flags |= DIR_SHOW_IGNORED;
 		topts.dir->exclude_per_dir = ".gitignore";
+		topts.apply_sparse = opts->apply_sparse;
 		tree = parse_tree_indirect(old->commit->object.sha1);
 		init_tree_desc(&trees[0], tree->buffer, tree->size);
 		tree = parse_tree_indirect(new->commit->object.sha1);

@@ -594,6 +596,8 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 		OPT_BOOLEAN('m', "merge", &opts.merge, "merge"),
 		OPT_STRING(0, "conflict", &conflict_style, "style",
 			   "conflict style (merge or diff3)"),
+		OPT_SET_INT(0, "sparse", &opts.apply_sparse,
+			    "apply sparse checkout filter", 1),
 		OPT_END(),
 	};
 	int has_dash_dash;

@@ -43,7 +43,7 @@ static const char * const builtin_merge_usage[] = {
 
 static int show_diffstat = 1, option_log, squash;
 static int option_commit = 1, allow_fast_forward = 1;
-static int allow_trivial = 1, have_message;
+static int allow_trivial = 1, have_message, apply_sparse;
 static struct strbuf merge_msg;
 static struct commit_list *remoteheads;
 static unsigned char head[20], stash[20];

@@ -172,6 +172,7 @@ static struct option builtin_merge_options[] = {
 	OPT_CALLBACK('m', "message", &merge_msg, "message",
 		"message to be used for the merge commit (if any)",
 		option_parse_message),
+	OPT_SET_INT(0, "sparse", &apply_sparse, "apply sparse checkout filter", 1),
 	OPT__VERBOSITY(&verbosity),
 	OPT_END()
 };

@@ -494,6 +495,7 @@ static int read_tree_trivial(unsigned char *common, unsigned char *head,
 	opts.verbose_update = 1;
 	opts.trivial_merges_only = 1;
 	opts.merge = 1;
+	opts.apply_sparse = apply_sparse;
 	trees[nr_trees] = parse_tree_indirect(common);
 	if (!trees[nr_trees++])
 		return -1;

@@ -646,6 +648,7 @@ static int checkout_fast_forward(unsigned char *head, unsigned char *remote)
 	opts.verbose_update = 1;
 	opts.merge = 1;
 	opts.fn = twoway_merge;
+	opts.apply_sparse = apply_sparse;
 
 	trees[nr_trees] = parse_tree_indirect(head);
 	if (!trees[nr_trees++])

@@ -20,6 +20,7 @@ strategy_args= diffstat= no_commit= squash= no_ff= log_arg= verbosity=
 curr_branch=$(git symbolic-ref -q HEAD)
 curr_branch_short=$(echo "$curr_branch" | sed "s|refs/heads/||")
 rebase=$(git config --bool branch.$curr_branch_short.rebase)
+sparse=
 while :
 do
 	case "$1" in

@@ -65,6 +66,9 @@ do
 	--no-r|--no-re|--no-reb|--no-reba|--no-rebas|--no-rebase)
 		rebase=false
 		;;
+	--sparse)
+		sparse=--sparse
+		;;
 	-h|--h|--he|--hel|--help)
 		usage
 		;;

@@ -201,5 +205,5 @@ merge_name=$(git fmt-merge-msg $log_arg <"$GIT_DIR/FETCH_HEAD") || exit
 test true = "$rebase" &&
 	exec git-rebase $diffstat $strategy_args --onto $merge_head \
 	${oldremoteref:-$merge_head}
-exec git-merge $diffstat $no_commit $squash $no_ff $log_arg $strategy_args \
+exec git-merge $sparse $diffstat $no_commit $squash $no_ff $log_arg $strategy_args \
 	"$merge_name" HEAD $merge_head $verbosity

@@ -445,8 +445,9 @@ static int same(struct cache_entry *a, struct cache_entry *b)
  * When a CE gets turned into an unmerged entry, we
  * want it to be up-to-date
  */
-static int verify_uptodate(struct cache_entry *ce,
-		struct unpack_trees_options *o)
+static int verify_uptodate_1(struct cache_entry *ce,
+				   struct unpack_trees_options *o,
+				   const char *error_msg)
 {
 	struct stat st;
 

@@ -471,7 +472,13 @@ static int verify_uptodate(struct cache_entry *ce,
 	if (errno == ENOENT)
 		return 0;
 	return o->gently ? -1 :
-		error(ERRORMSG(o, not_uptodate_file), ce->name);
+		error(error_msg, ce->name);
+}
+
+static int verify_uptodate(struct cache_entry *ce,
+			   struct unpack_trees_options *o)
+{
+	return verify_uptodate_1(ce, o, ERRORMSG(o, not_uptodate_file));
 }
 
 static void invalidate_ce_path(struct cache_entry *ce, struct unpack_trees_options *o)

@@ -579,8 +586,9 @@ static int icase_exists(struct unpack_trees_options *o, struct cache_entry *dst,
  * We do not want to remove or overwrite a working tree file that
  * is not tracked, unless it is ignored.
  */
-static int verify_absent(struct cache_entry *ce, const char *action,
-			 struct unpack_trees_options *o)
+static int verify_absent_1(struct cache_entry *ce, const char *action,
+				 struct unpack_trees_options *o,
+				 const char *error_msg)
 {
 	struct stat st;
 

@@ -660,6 +668,11 @@ static int verify_absent(struct cache_entry *ce, const char *action,
 	}
 	return 0;
 }
+static int verify_absent(struct cache_entry *ce, const char *action,
+			 struct unpack_trees_options *o)
+{
+	return verify_absent_1(ce, action, o, ERRORMSG(o, would_lose_untracked));
+}
 
 static int merged_entry(struct cache_entry *merge, struct cache_entry *old,
 		struct unpack_trees_options *o)