diff-lib: skip paths outside prefix in oneway_diff()

diff-lib: skip paths outside prefix in oneway_diff()

Commit 8174627b3d (diff-lib: ignore paths that are outside $cwd if
--relative asked, 2021-08-22) taught run_diff_files() to skip entries
outside the requested prefix before processing them.

Do the same in oneway_diff(), which handles the diff-index code path.
The lower-level diff queue functions already reject such paths, but
checking here avoids unnecessary work and keeps them out of every
do_oneway_diff() code path.

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