localrepo: introduce a `_quick_access_changeid` property
Having faster access to `null` is cute… but limited. We want to speedup access
to more useful revision, like `.`. We start with turning the fast path for
`null` into something more generic.
Differential Revision: https://phab.mercurial-scm.org/D7488
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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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).