repoview: use the phasecache directly to determine mutable revisions
This speeds up no-change updates from 1.3s to 0.85s for my NetBSD test
repository.
Differential Revision: https://phab.mercurial-scm.org/D8670
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https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
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If you can't run the entire testsuite for some reason (it can be
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273ce12ad8f1` will report some style violations on a very old commit).