manifest: subclass the new `repository.imanifestlog` 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.

mercurial/manifest.py

@@ -2055,7 +2055,7 AnyManifestCtx = Union['manifestctx', 't
 AnyManifestDict = Union[manifestdict, treemanifest]
 
 
-class manifestlog:  # (repository.imanifestlog)
+class manifestlog(repository.imanifestlog):
     """A collection class representing the collection of manifest snapshots
     referenced by commits in the repository.