git-p4: Fix import of changesets with file deletions

git-p4: Fix import of changesets with file deletions

Commit 3a70cdfa42199e16d2d047c286431c4274d65b1a 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: Simon Hausmann <simon@lst.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>