Fold get_merge_bases_clean() into get_merge_bases()

Fold get_merge_bases_clean() into get_merge_bases()

Change get_merge_bases() to be able to clean up after itself if
needed by adding a cleanup parameter.

We don't need to save the flags and restore them afterwards anymore;
that was a leftover from before the flags were moved out of the
range used in revision.c.  clear_commit_marks() sets them to zero,
which is enough.

Signed-off-by: Rene Scharfe <redacted>
Acked-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>