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