checkheads: use "revnum" in the "allfuturecommon" set
The obsolete post-processing needs to know the extend of the pushed set. The
way it is implemented is... suboptimal. It build a full set of all nodes in the
pushset and it does so using changectx. We have much better API for this now.
The simplest is to use the existing lazy ancestors computation. That logic uses
revnum and not node (for good reason) so we start with updating the
post-processing code to handle a "allfuturecommon" set containing revision
numbers.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).