Redo "add test_cmp function for test scripts"

We had a handful test updates since we accepted 82ebb0b (add test_cmp
function for test scripts).  This fixes them up.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4201-shortlog.sh

@@ -50,6 +50,6 @@ test_expect_success 'shortlog wrapping' 'test_cmp expect out'
 git log HEAD > log
 GIT_DIR=non-existing git shortlog -w < log > out
 
-test_expect_success 'shortlog from non-git directory' 'diff -u expect out'
+test_expect_success 'shortlog from non-git directory' 'test_cmp expect out'
 
 test_done

t/t6031-merge-recursive.sh

@@ -42,7 +42,7 @@ test_expect_success 'mode change in both branches: expect conflict' '
 		echo "100755 $H 2	file2"
 		echo "100644 $H 3	file2"
 	) >expect &&
-	diff -u actual expect &&
+	test_cmp actual expect &&
 	test -x file2
 '