[PATCH 2/4] merge-base: "--is-ancestor A B"

[Junio C Hamano <gitster@pobox.com>]

In many scripted Porcelain commands, we find this idiom:

    if test "$(git rev-parse --verify A)" = "$(git merge-base A B)"
    then
    	... A is an ancestor of B ...
    fi

But you do not have to compute exact merge-base only to see if A is
an ancestor of B.  Give them a more direct way to use the underlying
machinery.
---
 builtin/merge-base.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/builtin/merge-base.c b/builtin/merge-base.c
index 4f30f1b..615aa04 100644
--- a/builtin/merge-base.c
+++ b/builtin/merge-base.c
@@ -70,6 +70,20 @@ static int handle_octopus(int count, const char **args, int reduce, int show_all
 	return 0;
 }
 
+static int show_is_ancestor(int argc, const char **argv)
+{
+	struct commit *one, *two;
+
+	if (argc != 2)
+		die("--is-ancestor takes exactly two commits");
+	one = get_commit_reference(argv[0]);
+	two = get_commit_reference(argv[1]);
+	if (in_merge_bases(one, two))
+		return 0;
+	else
+		return 1;
+}
+
 int cmd_merge_base(int argc, const char **argv, const char *prefix)
 {
 	struct commit **rev;
@@ -77,11 +91,14 @@ int cmd_merge_base(int argc, const char **argv, const char *prefix)
 	int show_all = 0;
 	int octopus = 0;
 	int reduce = 0;
+	int is_ancestor = 0;
 
 	struct option options[] = {
 		OPT_BOOLEAN('a', "all", &show_all, "output all common ancestors"),
 		OPT_BOOLEAN(0, "octopus", &octopus, "find ancestors for a single n-way merge"),
 		OPT_BOOLEAN(0, "independent", &reduce, "list revs not reachable from others"),
+		OPT_BOOLEAN(0, "is-ancestor", &is_ancestor,
+			    "is the first one ancestor of the other?"),
 		OPT_END()
 	};
 
@@ -89,6 +106,10 @@ int cmd_merge_base(int argc, const char **argv, const char *prefix)
 	argc = parse_options(argc, argv, prefix, options, merge_base_usage, 0);
 	if (!octopus && !reduce && argc < 2)
 		usage_with_options(merge_base_usage, options);
+	if (is_ancestor && (show_all | octopus | reduce))
+		die("--is-reachable cannot be used with other options");
+	if (is_ancestor)
+		return show_is_ancestor(argc, argv);
 	if (reduce && (show_all || octopus))
 		die("--independent cannot be used with other options");
 
-- 
1.7.12.293.g6aeebca