'``save`` returns a ``Deferred`` that fires with ``None`` when called
with a deployment that is the same as the already-saved deployment.'

def test_success_returned_for_unchanged_deployment(self):

    service = self.service(FilePath(self.mktemp()), None)
    old_saving = service.save(LATEST_TEST_DEPLOYMENT)
    def saved_old(ignored):
        new_saving = service.save(LATEST_TEST_DEPLOYMENT)
        new_saving.addCallback(self.assertEqual, None)
        return new_saving
    old_saving.addCallback(saved_old)
    return old_saving
