revlog-split: make sure the self._indexfile attribut is reset (issue6811)
Before this change, after a transaction committing a file split, a revlog
object would have its `self._indexfile` attribute desynchronised from the
actual file storing the data. If that same object is reused (as we do for the
manifest during clone bundles), this lead to the data being writting in the
wrong location and the repository to go corrupt.
We not properly reset the attribut when applicable and everything is back in
working order.
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).