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>

diff-lib.c

@@ -538,6 +538,11 @@ static int oneway_diff(const struct cache_entry * const *src,
 	if (!idx && !tree)
 		BUG("oneway_diff with neither idx nor tree");
 
+	if (revs->diffopt.prefix &&
+	    strncmp((idx ? idx : tree)->name, revs->diffopt.prefix,
+		    revs->diffopt.prefix_length))
+		return 0;
+
 	if (ce_path_match(revs->diffopt.repo->index,
 			  idx ? idx : tree,
 			  &revs->prune_data, NULL)) {