t3000: use test_cmp instead of diff

These ancient tests predate test_cmp.

While we're at it, let's switch to our usual "expected
before actual" order of arguments; this makes the diff
output "here's what is changed from expected" instead of the
reverse.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3000-ls-files-others.sh

@@ -42,7 +42,7 @@ test_expect_success \
 
 test_expect_success \
     'git ls-files --others should pick up symlinks.' \
-    'diff output expected1'
+    'test_cmp expected1 output'
 
 test_expect_success \
     'git ls-files --others --directory to show output.' \
@@ -51,6 +51,6 @@ test_expect_success \
 
 test_expect_success \
     'git ls-files --others --directory should not get confused.' \
-    'diff output expected2'
+    'test_cmp expected2 output'
 
 test_done