templatekw: cache mergestate even if merge is not ongoing
While playing with eBPF, I noticed .hg/merge/state{,2} files were tried
to open() for each revision. That's not healthy. Let's cache the "inactive"
state as well.
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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commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).