revision traversal: --remove-empty fix.

revision traversal: --remove-empty fix.

Marco Costalba reports that --remove-empty omits the commit that
created paths we are interested in.  try_to_simplify_commit()
logic was dropping a parent we introduced those paths against,
which I think is not what we meant.  Instead, this marks such
parent uninteresting.  The traversal does not go beyond that
parent as advertised, but we still say that the current commit
changed things from that parent.

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