exchange: use "served" repo filter to guess what the server will publish Previously, the second push in the added test would say "push would publish 2 changesets" because _checkpublish() was considering secret changesets to be "pushable" when push command had a --rev argument. Without --rev argument to push command, we already filter the repo properly. Differential Revision: https://phab.mercurial-scm.org/D10948