diff: handle relative paths in no-index

diff: handle relative paths in no-index

When diff-no-index is given a relative path to a file outside the
repository, it aborts with error. However, if the file is given
using an absolute path, the diff runs as expected. The two cases
should be treated the same.

Tests and commit message by Tim Henigan.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Tim Henigan <tim.henigan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>