git-p4: Fix git-p4 submit to include only changed files in the perforce submit template.

Parse the files section in the "p4 change -o" output and remove lines with file changes in unrelated depot paths.

Signed-off-by: Simon Hausmann <simon@lst.de>
Signed-off-by: Marius Storm-Olsen <marius@trolltech.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>