Teach "git diff" to handle filenames starting with '-'

It adds "--" to the git-diff.sh scripts, to keep any filenames that start
with a "-" from being confused with an option.

But in order to do that, it needs to teach git-diff-files to honor "--".

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>