Sorry, my previous message accidentally sent before I was done.  One more comment:

-----Original Message-----
From: Stefan Beller [mailto:sbeller@google.com]

+test_expect_failure '"checkout --recurse-submodules" needs -f to update
modifed submodule content' '
+	echo modified >submodule/second.t &&
+	test_must_fail git checkout --recurse-submodules HEAD^ &&
+	test_must_fail git diff-files --quiet submodule &&
+	git diff-files --quiet file &&
+	git checkout --recurse-submodules -f HEAD^ &&
+	git diff-files --quiet &&
+	git diff-index --quiet --cached HEAD &&
+	git checkout --recurse-submodules -f master &&
+	git diff-files --quiet &&
+	git diff-index --quiet --cached HEAD
+'

It might be worth adding some comments explaining why you expect these to fail.