t7406: avoid using test_must_fail for commands other than git

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7406-submodule-update.sh

@@ -606,7 +606,7 @@ test_expect_success 'submodule update - update=none in .git/config but --checkou
 	 git diff --name-only >out &&
 	 ! grep ^submodule$ out &&
 	 (cd submodule &&
-	  test_must_fail compare_head
+	  ! compare_head
 	 ) &&
 	 git config --unset submodule.submodule.update
 	)