testing: fix _timeout_factor
With `--debug`, `run-tests.py` set the timeout to 0... that breaks the logic in
`mercurial.testing`.
Differential Revision: https://phab.mercurial-scm.org/D10937
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