localrepo: add isfilecached to check filecache-ed property is already cached
isfilecached() encapsulates internal implementation of filecache-ed
property.
"name in repo.unfiltered().__dict__" or so can't be used for this
purpose, because corresponded entry in __dict__ might be discarded by
repo.invalidate(), repo.invalidatedirstate() or so (fsmonitor does so,
for example).
This patch makes isfilecached() return not only whether filecache-ed
property is already cached, but also already cached value (or None),
in order to avoid subsequent access to cached object via "repo.NAME",
which prevents main Mercurial procedure after reposetup() from
validating cache.
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
difficult on Windows), please at least run `contrib/check-code.py` on
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).