Treat D/F conflict entry more carefully in unpack-trees.c::threeway_merge()

Treat D/F conflict entry more carefully in unpack-trees.c::threeway_merge()

This fixes three buglets in threeway_merge() regarding D/F
conflict entries.

* After finishing with path D and handling path D/F, some stages
  have D/F conflict entry which are obviously non-NULL.  For the
  purpose of determining if the path D/F is missing in the
  ancestor, they should not be taken into account.

* D/F conflict entry is a marker to say "this stage does _not_
  have the path", so do not send them to keep_entry().

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