get_merge_bases(): always clean-up object flags

The callers of get_merge_bases() can choose to leave object flags
used during the merge-base traversal by passing cleanup=0 as a
parameter, but in practice a very few callers can afford to do so
(namely, "git merge-base"), as they need to compute merge base in
preparation for other processing of their own and they need to see
the object without contaminate flags.

Change the function signature of get_merge_bases_many() and
get_merge_bases() to drop the cleanup parameter, so that the
majority of the callers do not have to say ", 1" at the end.

Give a new get_merge_bases_many_dirty() API to support only a few
callers that know they do not need to spend cycles cleaning up the
object flags.

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

builtin/rev-parse.c

@@ -277,7 +277,7 @@ static int try_difference(const char *arg)
 			struct commit *a, *b;
 			a = lookup_commit_reference(sha1);
 			b = lookup_commit_reference(end);
-			exclude = get_merge_bases(a, b, 1);
+			exclude = get_merge_bases(a, b);
 			while (exclude) {
 				struct commit_list *n = exclude->next;
 				show_rev(REVERSED,