Add get_merge_bases_clean()

Add get_merge_bases_clean()

Add get_merge_bases_clean(), a wrapper for get_merge_bases() that cleans
up after doing its work and make get_merge_bases() NOT clean up.
Single-shot programs like git-merge-base can use the dirty and fast
version.

Also move the object flags used in get_merge_bases() out of the range
defined in revision.h.  This fixes the "66ae0c77...ced9456a
89719209...262a6ef7" test of the ... operator which is introduced with
the next patch.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <junkio@cox.net>