memctx: inherit from committablectx

This patch marks the start of having memctx inherit from committablectx,
thereby making it a full-fledged context that will eventually grow the ability
to perform diffing and also merging.

mercurial/context.py

@@ -1531,7 +1531,7 b' class workingfilectx(committablefilectx)'
         # invert comparison to reuse the same code path
         return fctx.cmp(self)
 
-class memctx(object):
+class memctx(committablectx):
     """Use memctx to perform in-memory commits via localrepo.commitctx().
 
     Revision information is supplied at initialization time while