diff-files: do not play --no-index games

diff-files: do not play --no-index games

Being able to say "git diff A B" outside a git repository and getting a
colourful version of "diff -u A B" may be nice, but such a cute hack
should not give bogus results to scripts that want to give two paths,
either or both of which happen to have been removed from the work tree,
to "git diff-files".

Signed-off-by: Junio C Hamano <gitster@pobox.com>