memfilectx: inherit from committablefilectx

This patch marks the beginning of having memfilectx become a full-fledged file
contex so that we can handle diffing and merging.

mercurial/context.py

@@ -1586,7 +1586,7 b' class memctx(committablectx):'
         """commit context to the repo"""
         return self._repo.commitctx(self)
 
-class memfilectx(object):
+class memfilectx(committablefilectx):
     """memfilectx represents an in-memory file to commit.
 
     See memctx for more details.