t7406: fix call that was failing for the wrong reason

A test making use of test_must_fail was failing like this:
  fatal: ambiguous argument '|': unknown revision or path not in the working tree.
when the intent was to verify that a specific string was not found
in the output of the git diff command, i.e. that grep returned
non-zero.  Fix the test to do that.

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

t/t7406-submodule-update.sh

@@ -599,7 +599,8 @@ test_expect_success 'submodule update - update=none in .git/config but --checkou
 	 ) &&
 	 git diff --raw | grep "	submodule" &&
 	 git submodule update --checkout &&
-	 test_must_fail git diff --raw \| grep "	submodule" &&
+	 git diff --raw >out &&
+	 ! grep "	submodule" out &&
 	 (cd submodule &&
 	  test_must_fail compare_head
 	 ) &&