phases: use a more generic way to trigger a phases computation for perf
Querying the tip most revision will require the cache to warm the same as
calling the dedicated method. This avoid using a method that is mostly meant for
internal use and will be renamed in a coming changesets.
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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
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273ce12ad8f1` will report some style violations on a very old commit).