Tests: use test_cmp instead of diff where possible

Several old tests were written before test_cmp was introduced, convert
these to test_cmp.

If were are at it, fix the order of the arguments where necessary to
make expected come first, so the command shows how the test result
deviates from the correct output.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1100-commit-tree-options.sh

@@ -40,6 +40,6 @@ test_expect_success \
 
 test_expect_success \
     'compare commit' \
-    'diff expected commit'
+    'test_cmp expected commit'
 
 test_done