[PATCH] t7406: Make a test_must_fail call fail for the right reason

[Elijah Newren <newren@gmail.com>]

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.
---
 t/t7406-submodule-update.sh | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/t/t7406-submodule-update.sh b/t/t7406-submodule-update.sh
index f604ef7a72..7be8b59569 100755
--- a/t/t7406-submodule-update.sh
+++ b/t/t7406-submodule-update.sh
@@ -599,7 +599,7 @@ 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 | test_must_fail grep "	submodule" &&
 	 (cd submodule &&
 	  test_must_fail compare_head
 	 ) &&
-- 
2.18.0.548.gcd835b18f7