subrepos: add missing self argument to abstractsubrepo.checknested

mercurial/subrepo.py

@@ -220,7 +220,7 b' class abstractsubrepo(object):'
         """
         raise NotImplementedError
 
-    def checknested(path):
+    def checknested(self, path):
         """check if path is a subrepository within this repository"""
         return False