merge-base: die with an error message if not passed a commit ref

Before this patch "git merge-base" just exited with error code 1
and without an error message in case it was passed a ref to an
object that is not a commit (for example a tree).

This patch makes it "die" in this case with an error message.

While at it, this patch also refactors the code to get the
commit reference from an argument into a new
"get_commit_reference" function.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-merge-base.c

diff --git a/builtin-merge-base.c b/builtin-merge-base.c
index 1cb2925..3382b13 100644
--- a/builtin-merge-base.c
+++ b/builtin-merge-base.c
@@ -22,10 +22,23 @@
 static const char merge_base_usage[] =
 "git merge-base [--all] <commit-id> <commit-id>";
 
+static struct commit *get_commit_reference(const char *arg)
+{
+	unsigned char revkey[20];
+	struct commit *r;
+
+	if (get_sha1(arg, revkey))
+		die("Not a valid object name %s", arg);
+	r = lookup_commit_reference(revkey);
+	if (!r)
+		die("Not a valid commit name %s", arg);
+
+	return r;
+}
+
 int cmd_merge_base(int argc, const char **argv, const char *prefix)
 {
 	struct commit *rev1, *rev2;
-	unsigned char rev1key[20], rev2key[20];
 	int show_all = 0;
 
 	git_config(git_default_config, NULL);
@@ -40,13 +53,8 @@
 	}
 	if (argc != 3)
 		usage(merge_base_usage);
-	if (get_sha1(argv[1], rev1key))
-		die("Not a valid object name %s", argv[1]);
-	if (get_sha1(argv[2], rev2key))
-		die("Not a valid object name %s", argv[2]);
-	rev1 = lookup_commit_reference(rev1key);
-	rev2 = lookup_commit_reference(rev2key);
-	if (!rev1 || !rev2)
-		return 1;
+	rev1 = get_commit_reference(argv[1]);
+	rev2 = get_commit_reference(argv[2]);
+
 	return show_merge_base(rev1, rev2, show_all);
 }