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).

hgext/git/gitlog.py

@@ -21,6 +21,9 from mercurial import (
     manifest,
     pycompat,
 )
+from mercurial.interfaces import (
+    repository,
+)
 from mercurial.utils import stringutil
 from . import (
     gitutil,
@@ -471,8 +474,7 class manifestlog(baselog):
         return gitmanifest.gittreemanifestctx(self.gitrepo, t)
 
 
-# @interfaceutil.implementer(repository.ifilestorage)
-class filelog(baselog):
+class filelog(baselog, repository.ifilestorage):
     def __init__(self, gr, db, path):
         super(filelog, self).__init__(gr, db)
         assert isinstance(path, bytes)

hgext/sqlitestore.py

@@ -312,8 +312,7 class sqliteproblem(repository.iverifypr
     node = attr.ib(default=None, type=Optional[bytes])
 
 
[e-mail](repository.ifilestorage)
-class sqlitefilestore:
+class sqlitefilestore(repository.ifilestorage):
     """Implements storage for an individual tracked path."""
 
     def __init__(self, db, path, compression):

mercurial/filelog.py

@@ -28,7 +28,7 from .revlogutils import (
 )
 
 
-class filelog:  # (repository.ifilestorage)
+class filelog(repository.ifilestorage):
     _revlog: revlog.revlog
     nullid: bytes
     _fix_issue6528: bool

tests/simplestorerepo.py

@@ -40,7 +40,6 from mercurial import (
 )
 from mercurial.interfaces import (
     repository,
-    util as interfaceutil,
 )
 from mercurial.utils import (
     cborutil,
@@ -96,8 +95,7 class simplefilestoreproblem(repository.
     node = attr.ib(default=None, type=Optional[bytes])
 
 
[e-mail](repository.ifilestorage)
-class filestorage:
+class filestorage(repository.ifilestorage):
     """Implements storage for a tracked path.
 
     Data is stored in the VFS in a directory corresponding to the tracked