subrepo: use vfs.dirname instead of os.path.dirname

This patch uses "wvfs of the parent repository" ('pwvfs') instead of
'wvfs' of own repository, because 'self._path' is the path to this
subrepository as seen from the parent repository.

mercurial/subrepo.py

@@ -1206,7 +1206,8 b' class svnsubrepo(abstractsubrepo):'
 
         self.wvfs.rmtree(forcibly=True)
         try:
-            self._ctx.repo().wvfs.removedirs(os.path.dirname(self._path))
+            pwvfs = self._ctx.repo().wvfs
+            pwvfs.removedirs(pwvfs.dirname(self._path))
         except OSError:
             pass