t7502: use diff.noprefix for --verbose test

t7502: use diff.noprefix for --verbose test

To check that "status -v" respects diff config, we set
"color.diff" and look at the output of "status". We could
equally well use any diff config. Since color output depends
on a lot of other factors (like whether stdout is a tty, and
how we interpret "always"), let's use a more mundane option.

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