merge-recursive: Allow make_room_for_path() to remove D/F entries

merge-recursive: Allow make_room_for_path() to remove D/F entries

If there were several files conflicting below a directory corresponding
to a D/F conflict, and the file of that D/F conflict is in the way, we
want it to be removed.  Since files of D/F conflicts are handled last,
they can be reinstated later and possibly with a new unique name.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>