[PATCH] optionally disable overwriting of ignored files

@@ -450,6 +450,12 @@ core.excludesfile::
 	to the value of `$HOME` and "{tilde}user/" to the specified user's
 	home directory.  See linkgit:gitignore[5].
 
+core.overwriteignored::
+	Untracked files can get in the way of merge or checkout. If
+	overwriteignored is enabled, such files will be overwritten
+	to let fast-forward merges and checkouts succeed. Enabled
+	by default.
+
 core.editor::
 	Commands such as `commit` and `tag` that lets you edit
 	messages by launching an editor uses the value of this

@@ -389,6 +389,11 @@ static int merge_working_tree(struct checkout_opts *opts,
 		topts.gently = opts->merge && old->commit;
 		topts.verbose_update = !opts->quiet;
 		topts.fn = twoway_merge;
+		if (overwrite_ignored) {
+			topts.dir = xcalloc(1, sizeof(*topts.dir));
+			topts.dir->flags |= DIR_SHOW_IGNORED;
+			topts.dir->exclude_per_dir = ".gitignore";
+		}
 		tree = parse_tree_indirect(old->commit ?
 					   old->commit->object.sha1 :
 					   (unsigned char *)EMPTY_TREE_SHA1_BIN);

@@ -664,6 +669,8 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 		OPT_SET_INT('3', "theirs", &opts.writeout_stage, "stage",
 			    3),
 		OPT_BOOLEAN('f', "force", &opts.force, "force"),
+		OPT_BOOLEAN('i', "overwrite-ignored", &overwrite_ignored,
+		  "allow explicitly ignored files to be overwritten"),
 		OPT_BOOLEAN('m', "merge", &opts.merge, "merge"),
 		OPT_STRING(0, "conflict", &conflict_style, "style",
 			   "conflict style (merge or diff3)"),

@@ -185,6 +185,8 @@ static struct option builtin_merge_options[] = {
 		"allow fast-forward (default)"),
 	OPT_BOOLEAN(0, "ff-only", &fast_forward_only,
 		"abort if fast-forward is not possible"),
+	OPT_BOOLEAN('i', "overwrite-ignored", &overwrite_ignored,
+	  "allow explicitly ignored files to be overwritten"),
 	OPT_RERERE_AUTOUPDATE(&allow_rerere_auto),
 	OPT_CALLBACK('s', "strategy", &use_strategies, "strategy",
 		"merge strategy to use", option_parse_strategy),

@@ -682,6 +684,7 @@ int checkout_fast_forward(const unsigned char *head, const unsigned char *remote
 	struct unpack_trees_options opts;
 	struct tree_desc t[MAX_UNPACK_TREES];
 	int i, fd, nr_trees = 0;
+	struct dir_struct dir;
 	struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
 
 	refresh_cache(REFRESH_QUIET);

@@ -691,6 +694,12 @@ int checkout_fast_forward(const unsigned char *head, const unsigned char *remote
 	memset(&trees, 0, sizeof(trees));
 	memset(&opts, 0, sizeof(opts));
 	memset(&t, 0, sizeof(t));
+	memset(&dir, 0, sizeof(dir));
+	if (overwrite_ignored) {
+		dir.flags |= DIR_SHOW_IGNORED;
+		dir.exclude_per_dir = ".gitignore";
+		opts.dir = &dir;
+	}
 
 	opts.head_idx = 1;
 	opts.src_index = &the_index;

@@ -548,6 +548,7 @@ extern size_t packed_git_window_size;
 extern size_t packed_git_limit;
 extern size_t delta_base_cache_limit;
 extern int read_replace_refs;
+extern int overwrite_ignored;
 extern int fsync_object_files;
 extern int core_preload_index;
 extern int core_apply_sparse_checkout;

@@ -563,6 +563,9 @@ static int git_default_core_config(const char *var, const char *value)
 	if (!strcmp(var, "core.excludesfile"))
 		return git_config_pathname(&excludes_file, var, value);
 
+	if (!strcmp(var, "core.overwriteignored"))
+		overwrite_ignored = git_config_bool(var, value);
+
 	if (!strcmp(var, "core.whitespace")) {
 		if (!value)
 			return config_error_nonbool(var);

@@ -30,6 +30,7 @@ const char *apply_default_ignorewhitespace;
 int zlib_compression_level = Z_BEST_SPEED;
 int core_compression_level;
 int core_compression_seen;
+int overwrite_ignored = 1;
 int fsync_object_files;
 size_t packed_git_window_size = DEFAULT_PACKED_GIT_WINDOW_SIZE;
 size_t packed_git_limit = DEFAULT_PACKED_GIT_LIMIT;