discovery: introduce a partialdiscovery object
This object will ultimately gather the data about common, undecided and
missing revs in a single place and deal with most graph related computations.
The goal is both to clarify the algorithm and to help provides a simple and
clear API that can be reimplemented in Rust.
For now, we only moved the `common` set in the object. In this commit, some
direct access to the "private" `disco._common` attribute persist. They have
not been removed yet because we won't need to expose a full API identical to
`incrementalmissingancestors` and it seems simpler to access the attribute
directly until the replacement is in place.
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).