t4012: use test_cmp instead of cmp

This makes erroneous output slightly easier to see. We also
flip the argument order to match our usual style.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

t/t4012-diff-binary.sh

@@ -25,11 +25,11 @@ cat > expected <<\EOF
 EOF
 test_expect_success 'diff without --binary' \
 	'git diff | git apply --stat --summary >current &&
-	 cmp current expected'
+	 test_cmp expected current'
 
 test_expect_success 'diff with --binary' \
 	'git diff --binary | git apply --stat --summary >current &&
-	 cmp current expected'
+	 test_cmp expected current'
 
 # apply needs to be able to skip the binary material correctly
 # in order to report the line number of a corrupt patch.