memctx: create parent contexts using "repo[p]" syntax

I want to reduce dependence on basectx.__new__() and move that code
over to repo.__getitem__().

Differential Revision: https://phab.mercurial-scm.org/D2968

mercurial/context.py

@@ -2284,7 +2284,7 b' class memctx(committablectx):'
         self._node = None
         parents = [(p or nullid) for p in parents]
         p1, p2 = parents
-        self._parents = [changectx(self._repo, p) for p in (p1, p2)]
+        self._parents = [self._repo[p] for p in (p1, p2)]
         files = sorted(set(files))
         self._files = files
         if branch is not None: