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>

bisect.c

@@ -776,7 +776,7 @@ static void check_merge_bases(int no_checkout)
 	int rev_nr;
 	struct commit **rev = get_bad_and_good_commits(&rev_nr);
 
-	result = get_merge_bases_many(rev[0], rev_nr - 1, rev + 1, 1);
+	result = get_merge_bases_many(rev[0], rev_nr - 1, rev + 1);
 
 	for (; result; result = result->next) {
 		const unsigned char *mb = result->item->object.sha1;