receive-pack: use in_merge_bases() for fast-forward check

The original computed merge-base between the old commit and the new
commit and checked if the old commit was a merge base between them,
in order to make sure we are fast-forwarding.

Instead, call in_merge_bases(old, new) which does the same.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/receive-pack.c

@@ -478,7 +478,6 @@ static const char *update(struct command *cmd)
 	    !prefixcmp(name, "refs/heads/")) {
 		struct object *old_object, *new_object;
 		struct commit *old_commit, *new_commit;
-		struct commit_list *bases, *ent;
 
 		old_object = parse_object(old_sha1);
 		new_object = parse_object(new_sha1);
@@ -491,12 +490,7 @@ static const char *update(struct command *cmd)
 		}
 		old_commit = (struct commit *)old_object;
 		new_commit = (struct commit *)new_object;
-		bases = get_merge_bases(old_commit, new_commit, 1);
-		for (ent = bases; ent; ent = ent->next)
-			if (!hashcmp(old_sha1, ent->item->object.sha1))
-				break;
-		free_commit_list(bases);
-		if (!ent) {
+		if (!in_merge_bases(old_commit, new_commit)) {
 			rp_error("denying non-fast-forward %s"
 				 " (you should pull first)", name);
 			return "non-fast-forward";