merge-base: avoid unnecessary postprocessing.
merge-base: avoid unnecessary postprocessing.

When we have only one merge-base candidates in the result list,
there is no point going back to mark the reachable commits
again.  And that is the most common case, so try not to waste
time on it.  Suggested by Linus.

Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git