Operations on unborn branch

@@ -225,6 +225,21 @@ static void die_if_unmerged_cache(int reset_type)
 
 }
 
+static struct object *lookup_commit_or_tree(const char *rev) {
+	unsigned char sha1[20];
+	struct commit *commit;
+	struct tree *tree;
+	if (get_sha1_treeish(rev, sha1))
+		die(_("Failed to resolve '%s' as a valid ref."), rev);
+	commit = lookup_commit_reference_gently(sha1, 1);
+	if (commit)
+		return (struct object *) commit;
+	tree = parse_tree_indirect(sha1);
+	if (tree)
+		return (struct object *) tree;
+	die(_("Could not parse object '%s'."), rev);
+}
+
 int cmd_reset(int argc, const char **argv, const char *prefix)
 {
 	int i = 0, reset_type = NONE, update_ref_status = 0, quiet = 0;

@@ -232,7 +247,8 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 	const char *rev = "HEAD";
 	unsigned char sha1[20], *orig = NULL, sha1_orig[20],
 				*old_orig = NULL, sha1_old_orig[20];
-	struct commit *commit;
+	struct object *object;
+	struct commit *commit = NULL;
 	struct strbuf msg = STRBUF_INIT;
 	const struct option options[] = {
 		OPT__QUIET(&quiet, N_("be quiet, only report errors")),

@@ -276,7 +292,7 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 		 * Otherwise, argv[i] could be either <rev> or <paths> and
 		 * has to be unambiguous.
 		 */
-		else if (!get_sha1_committish(argv[i], sha1)) {
+		else if (!get_sha1_treeish(argv[i], sha1)) {
 			/*
 			 * Ok, argv[i] looks like a rev; it should not
 			 * be a filename.

@@ -289,19 +305,12 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 		}
 	}
 
-	if (get_sha1_committish(rev, sha1))
-		die(_("Failed to resolve '%s' as a valid ref."), rev);
-
-	/*
-	 * NOTE: As "git reset $treeish -- $path" should be usable on
-	 * any tree-ish, this is not strictly correct. We are not
-	 * moving the HEAD to any commit; we are merely resetting the
-	 * entries in the index to that of a treeish.
-	 */
-	commit = lookup_commit_reference(sha1);
-	if (!commit)
-		die(_("Could not parse object '%s'."), rev);
-	hashcpy(sha1, commit->object.sha1);
+	object = lookup_commit_or_tree(rev);
+	if (object->type == OBJ_COMMIT)
+		commit = (struct commit*) object;
+	else if (reset_type == SOFT)
+		die(_("--soft requires a commit, which '%s' is not"), rev);
+	hashcpy(sha1, object->sha1);
 
 	if (patch_mode) {
 		if (reset_type != NONE)

@@ -347,23 +356,25 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 			die(_("Could not reset index file to revision '%s'."), rev);
 	}
 
-	/* Any resets update HEAD to the head being switched to,
-	 * saving the previous head in ORIG_HEAD before. */
-	if (!get_sha1("ORIG_HEAD", sha1_old_orig))
-		old_orig = sha1_old_orig;
-	if (!get_sha1("HEAD", sha1_orig)) {
-		orig = sha1_orig;
-		set_reflog_message(&msg, "updating ORIG_HEAD", NULL);
-		update_ref(msg.buf, "ORIG_HEAD", orig, old_orig, 0, MSG_ON_ERR);
+	if (commit) {
+		/* Any resets update HEAD to the head being switched to,
+		 * saving the previous head in ORIG_HEAD before. */
+		if (!get_sha1("ORIG_HEAD", sha1_old_orig))
+			old_orig = sha1_old_orig;
+		if (!get_sha1("HEAD", sha1_orig)) {
+			orig = sha1_orig;
+			set_reflog_message(&msg, "updating ORIG_HEAD", NULL);
+			update_ref(msg.buf, "ORIG_HEAD", orig, old_orig, 0, MSG_ON_ERR);
+		}
+		else if (old_orig)
+			delete_ref("ORIG_HEAD", old_orig, 0);
+		set_reflog_message(&msg, "updating HEAD", rev);
+		update_ref_status = update_ref(msg.buf, "HEAD", sha1, orig, 0, MSG_ON_ERR);
 	}
-	else if (old_orig)
-		delete_ref("ORIG_HEAD", old_orig, 0);
-	set_reflog_message(&msg, "updating HEAD", rev);
-	update_ref_status = update_ref(msg.buf, "HEAD", sha1, orig, 0, MSG_ON_ERR);
 
 	switch (reset_type) {
 	case HARD:
-		if (!update_ref_status && !quiet)
+		if (commit && !update_ref_status && !quiet)
 			print_new_head_line(commit);
 		break;
 	case SOFT: /* Nothing else to do. */

@@ -121,10 +121,6 @@ test_expect_success 'git log takes only commit-ish' '
 	git log 000000000
 '
 
-test_expect_success 'git reset takes only commit-ish' '
-	git reset 000000000
-'
-
 test_expect_success 'first tag' '
 	# create one tag 0000000000f8f
 	git tag -a -m j7cp83um v1.0.0

@@ -491,4 +491,30 @@ test_expect_success 'disambiguation (4)' '
 	test ! -f secondfile
 '
 
+test_expect_success 'reset to tree does not update HEAD' '
+	git reset --hard HEAD &&
+	rev_before=$(git rev-parse HEAD) &&
+	git reset HEAD^^{tree} &&
+	test $(git rev-parse HEAD) == $rev_before
+'
+
+test_expect_success 'reset to tree' '
+	# for simpler tests, drop last commit containing added files
+	git reset --hard HEAD^ &&
+	git reset HEAD^^{tree} &&
+	git diff --cached HEAD^ --exit-code &&
+	git diff HEAD --exit-code
+'
+
+test_expect_success 'reset --hard to tree' '
+	git reset --hard &&
+	git reset --hard HEAD^^{tree} &&
+	git diff --cached HEAD^ --exit-code &&
+	git diff HEAD^ --exit-code
+'
+
+test_expect_success 'reset to tree not allowed with --soft}' '
+	test_must_fail git reset --soft HEAD^^{tree}
+'
+
 test_done

@@ -229,7 +229,10 @@ static struct object *lookup_commit_or_tree(const char *rev) {
 	unsigned char sha1[20];
 	struct commit *commit;
 	struct tree *tree;
-	if (get_sha1_treeish(rev, sha1))
+	if (!strcmp(rev, "HEAD") && get_sha1("HEAD", sha1)) {
+		// unborn branch: reset to empty tree
+		hashcpy(sha1, EMPTY_TREE_SHA1_BIN);
+	} else if (get_sha1_treeish(rev, sha1))
 		die(_("Failed to resolve '%s' as a valid ref."), rev);
 	commit = lookup_commit_reference_gently(sha1, 1);
 	if (commit)

@@ -292,7 +295,8 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 		 * Otherwise, argv[i] could be either <rev> or <paths> and
 		 * has to be unambiguous.
 		 */
-		else if (!get_sha1_treeish(argv[i], sha1)) {
+		else if (!strcmp(argv[i], "HEAD") ||
+			 !get_sha1_treeish(argv[i], sha1)) {
 			/*
 			 * Ok, argv[i] looks like a rev; it should not
 			 * be a filename.

@@ -315,7 +319,7 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 	if (patch_mode) {
 		if (reset_type != NONE)
 			die(_("--patch is incompatible with --{hard,mixed,soft}"));
-		return interactive_reset(rev, argv + i, prefix);
+		return interactive_reset(sha1_to_hex(sha1), argv + i, prefix);
 	}
 
 	/* git reset tree [--] paths... can be used to

@@ -0,0 +1,52 @@
+#!/bin/sh
+
+test_description='git reset should work on unborn branch'
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	echo a >a &&
+	echo b >b
+'
+
+test_expect_success 'reset' '
+	git add a b &&
+	git reset &&
+	test "$(git ls-files)" == ""
+'
+
+test_expect_success 'reset HEAD' '
+	rm .git/index &&
+	git add a b &&
+	git reset HEAD &&
+	test "$(git ls-files)" == ""
+'
+
+test_expect_success 'reset $file' '
+	rm .git/index &&
+	git add a b &&
+	git reset a &&
+	test "$(git ls-files)" == "b"
+'
+
+test_expect_success 'reset -p' '
+	rm .git/index &&
+	git add a &&
+	echo y | git reset -p &&
+	test "$(git ls-files)" == ""
+'
+
+test_expect_success 'reset --soft not allowed' '
+	rm .git/index &&
+	git add a &&
+	test_must_fail git reset --soft
+'
+
+test_expect_success 'reset --hard' '
+	rm .git/index &&
+	git add a &&
+	git reset --hard &&
+	test "$(git ls-files)" == "" &&
+	test_path_is_missing a
+'
+
+test_done