fncache: add attribute to check whether we're using dotencode

This will make it easy to know if we can use the Rust implementation that
doesn't support older forms of encoding.

mercurial/store.py

@@ -1122,6 +1122,7 class _fncachevfs(vfsmod.proxyvfs):
         vfsmod.proxyvfs.__init__(self, vfs)
         self.fncache: fncache = fnc
         self.encode = encode
+        self.uses_dotencode = encode is _pathencode
 
     def __call__(self, path, mode=b'r', *args, **kw):
         encoded = self.encode(path)