Clients will send an empty pack file if they are only creating new references and have no objects to upload. Make gotd handle this and add a regression test which triggers the bug. Problem found by op@. The new regression test caught an unrelated issue where the client connection was left lingering after references had been updated, which made 'got send' followed by 'got clone -l' fail with the connection limit configured for the test suite (just one connection is allowed at a time). Fix this as well. ok op@

ok op, jamsek

ok op@

Previously the gotd tests were only running fresh clones.