rev-parse: Add support for the ^! and ^@ syntax

Those shorthands are explained in the rev-parse documentation but were not
actually supported by rev-parse itself.

gitk internally uses rev-parse to interpret its command line arguments, and
being able to use these "limit with parents" syntax is handy there.

Signed-off-by: Björn Steinbrink <B.Steinbrink@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-rev-parse.c

@@ -241,6 +241,36 @@ static int try_difference(const char *arg)
 	return 0;
 }
 
+static int try_parent_shorthands(const char *arg)
+{
+	char *dotdot;
+	unsigned char sha1[20];
+	struct commit *commit;
+	struct commit_list *parents;
+	int parents_only;
+
+	if ((dotdot = strstr(arg, "^!")))
+		parents_only = 0;
+	else if ((dotdot = strstr(arg, "^@")))
+		parents_only = 1;
+
+	if (!dotdot || dotdot[2])
+		return 0;
+
+	*dotdot = 0;
+	if (get_sha1(arg, sha1))
+		return 0;
+
+	if (!parents_only)
+		show_rev(NORMAL, sha1, arg);
+	commit = lookup_commit_reference(sha1);
+	for (parents = commit->parents; parents; parents = parents->next)
+		show_rev(parents_only ? NORMAL : REVERSED,
+				parents->item->object.sha1, arg);
+
+	return 1;
+}
+
 static int parseopt_dump(const struct option *o, const char *arg, int unset)
 {
 	struct strbuf *parsed = o->value;
@@ -573,6 +603,8 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 		/* Not a flag argument */
 		if (try_difference(arg))
 			continue;
+		if (try_parent_shorthands(arg))
+			continue;
 		name = arg;
 		type = NORMAL;
 		if (*arg == '^') {

t/t6101-rev-parse-parents.sh

@@ -28,6 +28,8 @@ test_expect_success 'final^1^2 != final^1^1' "test $(git rev-parse final^1^2) !=
 test_expect_success 'final^1^3 not valid' "if git rev-parse --verify final^1^3; then false; else :; fi"
 test_expect_success '--verify start2^1' 'test_must_fail git rev-parse --verify start2^1'
 test_expect_success '--verify start2^0' 'git rev-parse --verify start2^0'
+test_expect_success 'final^1^@ = final^1^1 final^1^2' "test \"$(git rev-parse final^1^@)\" = \"$(git rev-parse final^1^1 final^1^2)\""
+test_expect_success 'final^1^! = final^1 ^final^1^1 ^final^1^2' "test \"$(git rev-parse final^1^\!)\" = \"$(git rev-parse final^1 ^final^1^1 ^final^1^2)\""
 
 test_expect_success 'repack for next test' 'git repack -a -d'
 test_expect_success 'short SHA-1 works' '