rust-dirstatemap: remove additional lookups in dirstatemap

rust-dirstatemap: remove additional lookups in dirstatemap

We're copying this shortcut from the Python implementation, pretty standard
for this codebase.

Differential Revision: https://phab.mercurial-scm.org/D7117

--- a/mercurial/dirstate.py	Thu Oct 17 21:37:21 2019 +0200
+++ b/mercurial/dirstate.py	Wed Oct 16 10:14:26 2019 +0200
@@ -1784,6 +1784,10 @@
             if parents and not self._dirtyparents:
                 self.setparents(*parents)
 
+            self.__contains__ = self._rustmap.__contains__
+            self.__getitem__ = self._rustmap.__getitem__
+            self.get = self._rustmap.get
+
         def write(self, st, now):
             parents = self.parents()
             st.write(self._rustmap.write(parents[0], parents[1], now))