merge-recursive: do not use on-file index when not needed.

merge-recursive: do not use on-file index when not needed.

This revamps the merge-recursive implementation following the
outline in:

	Message-ID: <7v8xgileza.fsf@assigned-by-dhcp.cox.net>

There is no need to write out the index until the very end just
once from merge-recursive.  Also there is no need to write out
the resulting tree object for the simple case of merging with a
single merge base.

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