tests: use "test_cmp", not "diff", when verifying the result

In tests, call test_cmp rather than raw diff where possible (i.e. if
the output does not go to a pipe), to allow the use of, say, 'cmp'
when the default 'diff -u' is not compatible with a vendor diff.

When that is not possible, use $DIFF, as set in GIT-BUILD-OPTIONS.

Signed-off-by: Gary V. Vaughan <gary@thewrittenword.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9400-git-cvsserver-server.sh

@@ -435,7 +435,7 @@ test_expect_success 'cvs update (-p)' '
     rm -f failures &&
     for i in merge no-lf empty really-empty; do
         GIT_CONFIG="$git_config" cvs update -p "$i" >$i.out
-        diff $i.out ../$i >>failures 2>&1
+	test_cmp $i.out ../$i >>failures 2>&1
     done &&
     test -z "$(cat failures)"
 '