localrepo: remove unnecessary check of instance

The refactoring of all the context objects allows us to simply pass a basectx
to the __new__ constructor and have it return the same object without
allocating new memory.

mercurial/localrepo.py

@@ -1457,13 +1457,7 b' class localrepository(object):'
                     del mf[fn]
             return mf
 
-        if isinstance(node1, context.changectx):
-            ctx1 = node1
-        else:
             ctx1 = self[node1]
-        if isinstance(node2, context.changectx):
-            ctx2 = node2
-        else:
             ctx2 = self[node2]
 
         working = ctx2.rev() is None