branchcache: explicitly track inheritence "state" We move from a binary "dirty" flag to a three value "state": "clean", "inherited", "dirty". The "inherited" means that the branch cache is not only "clean", but it is a duplicate of its parent filter. If a branch cache is "inherited", we can non only skip writing its value on disk, but it is a good idea to delete any stale value on disk, as those will just waste time (and possibly induce bug) in the future. We only do this in the update related to transaction or explicit cache update (e.g `hg debugupdatecache`). Deleting the file when we simply detected a stall cache during a read only operation seems more dangerous. We rename `copy` to `inherit_for` to clarify we associate a stronger semantic to the operation.

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Our full contribution guidelines are in our wiki, please see:

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https://www.mercurial-scm.org/wiki/ContributingChanges

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If you just want a checklist to follow, you can go straight to

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https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist

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If you can't run the entire testsuite for some reason (it can be

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difficult on Windows), please at least run `contrib/check-code.py` on

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any files you've modified and run `python contrib/check-commit` on any

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commits you've made (for example, `python contrib/check-commit

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273ce12ad8f1` will report some style violations on a very old commit).