Use the modern syntax of git-diff-files in t2002-checkout-cache-u.sh

As a nice side effect it also fixes t2002-checkout-cache-u.sh on FreeBSD 4,
/bin/sh of which has problems interpreting "! command" construction.
Signed-off-by: NAlex Riesen <raa.lkml@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t2002-checkout-cache-u.sh

@@ -21,13 +21,13 @@ test_expect_success \
 rm -f path0 &&
 git read-tree $t &&
 git checkout-index -f -a &&
-! git diff-files | diff - /dev/null'
+test_must_fail git diff-files --exit-code'
 
 test_expect_success \
 'with -u, git checkout-index picks up stat information from new files.' '
 rm -f path0 &&
 git read-tree $t &&
 git checkout-index -u -f -a &&
-git diff-files | diff - /dev/null'
+git diff-files --exit-code'
 
 test_done