t7406: simplify by using diff --name-only instead of diff --raw

We can get rid of some quoted tabs and make a few tests slightly easier
to read and edit by just asking for the names of the files modified,
since that's all these tests were interested in anyway.

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

t/t7406-submodule-update.sh

@@ -579,9 +579,9 @@ test_expect_success 'submodule update - update=none in .git/config' '
 	  git checkout master &&
 	  compare_head
 	 ) &&
-	 git diff --raw | grep "	submodule" &&
+	 git diff --name-only | grep ^submodule$ &&
 	 git submodule update &&
-	 git diff --raw | grep "	submodule" &&
+	 git diff --name-only | grep ^submodule$ &&
 	 (cd submodule &&
 	  compare_head
 	 ) &&
@@ -597,10 +597,10 @@ test_expect_success 'submodule update - update=none in .git/config but --checkou
 	  git checkout master &&
 	  compare_head
 	 ) &&
-	 git diff --raw | grep "	submodule" &&
+	 git diff --name-only | grep ^submodule$ &&
 	 git submodule update --checkout &&
-	 git diff --raw >out &&
-	 ! grep "	submodule" out &&
+	 git diff --name-only >out &&
+	 ! grep ^submodule$ out &&
 	 (cd submodule &&
 	  test_must_fail compare_head
 	 ) &&