infinitepush: consider hidden revisions when computing backedup state

If a backup head is hidden (e.g. by rebase), then all its ancestor commits
appear as not-backed-up, even though they are.

Consider hidden commits as part of determining whether a revset is backed up or
not, and then filter out the hidden changesets afterwards.

Test Plan:
Add a new test case.

Differential Revision: https://phab.mercurial-scm.org/D1613