lib-submodule-update: teach test_submodule_content the -C <dir> flag

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/lib-submodule-update.sh

@@ -193,6 +193,11 @@ test_superproject_content () {
 # Test that the given submodule at path "$1" contains the content according
 # to the submodule commit recorded in the superproject's commit "$2"
 test_submodule_content () {
+	if test x"$1" = "x-C"
+	then
+		cd "$2"
+		shift; shift;
+	fi
 	if test $# != 2
 	then
 		echo "test_submodule_content needs two arguments"