Shuqi Liang wrote:

+test_expect_success 'diff-files with pathspec inside sparse definition' '
+	init_repos &&
+
+	write_script edit-contents <<-\EOF &&
+	echo text >>"$1"
+	EOF
+
+	run_on_all ../edit-contents deep/a &&
+
+	test_all_match git diff-files &&
+
+	test_all_match git diff-files deep/a &&
+
+	# test wildcard
+	test_all_match git diff-files "deep/*"

You added the '--' separator below, but not here. Was that intentional, or
should these have it as well? It doesn't make much of a practical difference
in this case, but it would be nice to remain consistent across all tests of
'diff-files' that you're adding. 

The same goes for some of the tests in patch 2 [1] ('sparse index is not
expanded: diff-files' and the perf tests).

[1] https://lore.kernel.org/git/20230508184652.4283-3-cheskaqiqi@gmail.com/ (local)

+'
+
+test_expect_success 'diff-files with pathspec outside sparse definition' '
+	init_repos &&
+
+	test_sparse_match git diff-files -- folder2/a &&
+
+	write_script edit-contents <<-\EOF &&
+	echo text >>"$1"
+	EOF
+
+	# The directory "folder1" is outside the cone of interest
+	# and may not exist in the sparse checkout repositories.
+	# Create it as needed, add file "folder1/a" there with
+	# contents that is different from the staged version.

nit: 'folder1/' *definitely* won't be present in the sparse-checkout
repositories, so "will not" would be more accurate than "may not".
Otherwise, this comment is clearer than before & better explains what's
going on here.

+	run_on_all mkdir -p folder1 &&
+	run_on_all cp a folder1/a &&
+
+	run_on_all ../edit-contents folder1/a &&
+	test_all_match git diff-files &&
+	test_all_match git diff-files -- folder1/a &&
+	test_all_match git diff-files -- "folder*/a"
+'
+
 test_done