branchcache: rework the `filteredhash` logic to be more generic
We now have a more flexible `key_hashes` tuple. We duplicated various logic in
the V2 and V3 version of the cache as the goal is to start changing the logic
for V3 in the next few changesets.
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
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commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).