filelog: subclass the new `repository.ifilestorage` Protocol class This is the same transformation as 3a90a6fd710d did for dirstate, but the CamelCase naming was already cleaned up here. See 4ef6dbc27a99 for the benefits of explicit subclassing. One thing to mention is that pytype gets confused if Protocol classes preceed a regular class in the superclass list, so the interface goes last in the git extension. (I didn't bother to verify that it would have been an issue here, rather it was something I noticed when making `repository.ipeerbase` a Protocol class locally before dropping it entirely earlier in this series.) One other thing is that PyCharm starts flagging `__len__()` and `hasnode()` on `hgext.git.gitlog.baselog` with: "Type of 'hasnode' is incompatible with 'ifilestorage'" No clue why; it's happy with the other 3 implementations (at least with these methods- simplestorerepo.py looks badly broken otherwise).