rust-revlog: add invalidation detection to `NodeTree` class
This will be useful for callers, such as `scmutil` who reuse a
`NodeTree` instance as a cache. They would otherwise get hard
errors if any mutation of the index occurred since instantiation.
This is something the C index does not provide.
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).