Lidong Yan [off-list ref] writes:

+test_expect_success 'diff -I<regex>: ignore all content changes' '
+	test_when_finished "git rm -f file1 file2 file3" &&
+	: >file1 &&
+	git add file1 &&
+	: >file2 &&
+	git add file2 &&
+	: >file3 &&
+	git add file3 &&
+
+	rm -f file1 file2 &&
+	mkdir file2 &&
+	echo "A" >file3 &&
+	A_hash=$(git hash-object -w file3) &&
+	echo "B" >file3 &&
+	B_hash=$(git hash-object -w file3) &&
+	cat <<-EOF | git update-index --index-info &&
+	100644 $A_hash 1	file3
+	100644 $B_hash 2	file3
+	EOF
+
+	test_diff_no_content_changes () {
+		git diff $1 --ignore-blank-lines -I".*" >actual &&
+		test_line_count = 3 actual &&
+		test_grep "file1" actual &&
+		test_grep "file2" actual &&
+		test_grep "file3" actual &&

I am puzzled by this part of the new test.

+		test_grep ! "diff --git" actual

The "test_grep !" is to make sure we do not leak the "patch" output
run in diff_flush_patch_quietly(), which is understandable, but in
the new world order that even raw, name-only, and name-status honor
"diff-from-contents" since b55e6d36 (diff: ensure consistent diff
behavior with ignore options, 2025-08-08), shouldn't we expect empty
"actual" that does not say file1/file2/file3 in it?

+	} &&
+	test_diff_no_content_changes "--raw" &&
+	test_diff_no_content_changes "--name-only" &&
+	test_diff_no_content_changes "--name-status" &&
+
+	: >actual &&
+	test_must_fail git diff --quiet -I".*" >actual &&
+	test_must_be_empty actual
+'
+
 # check_prefix <patch> <src> <dst>
 # check only lines with paths to avoid dependency on exact oid/contents
 check_prefix () {