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>

merge-recursive.c

@@ -1904,7 +1904,7 @@ int merge_recursive(struct merge_options *o,
 	}
 
 	if (!ca) {
-		ca = get_merge_bases(h1, h2, 1);
+		ca = get_merge_bases(h1, h2);
 		ca = reverse_commit_list(ca);
 	}