Commit 3a70cdfa made readP4Files abort quickly
when the changeset only contains files that are marked for deletion with an empty return
value, which caused the commit to not do anything.

This commit changes readP4Files to distinguish between files that need to be passed to p4
print and files that have no content ("deleted") and merge them in the returned
list.
Signed-off-by: NSimon Hausmann <simon@lst.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>