Arcyd: better error message when no diff on review

Make more useful comments when a review has been created or updated in
such a way that it has no difference against the base branch.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh

inspect review from
$ ./testbed/arcyd/test_shell.sh
    $ git checkout -b r/master/no_diff
    $ git commit --allow-empty -m 'empty!'
    $ git push -u origin r/master/no_diff
    $ git commit --allow-empty -m 'empty!'
    $ git push -u origin r/master/no_diff