diff: add --default-prefix option

diff: add --default-prefix option

You can change the output of prefixes with diff.noprefix and
diff.mnemonicprefix, but there's no easy way to override them from the
command-line. We do have "--no-prefix", but there's no way to get back
to the default prefix. So let's add an option to do that.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>