index: rename _fix_index() since it no longer fixes the index
Since c0d411ea6639 (index: drop support for negative indexes into the
index, 2018-07-20), it always returns the input (unless it raises).
Differential Revision: https://phab.mercurial-scm.org/D4334
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).