t3406: simplify an already simple test

When the merge backend was re-implemented on top of the interactive
backend, the output of rebase --merge changed a little.  This change
allowed this test to be simplified, though it wasn't noticed until now.
Simplify the testcase a little.

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

t/t3406-rebase-message.sh

@@ -18,11 +18,8 @@ test_expect_success 'setup' '
 '
 
 test_expect_success 'rebase -m' '
-	git rebase -m master >report &&
-	>expect &&
-	sed -n -e "/^Already applied: /p" \
-		-e "/^Committed: /p" report >actual &&
-	test_cmp expect actual
+	git rebase -m master >actual &&
+	test_must_be_empty actual
 '
 
 test_expect_success 'rebase against master twice' '