dirstate: make map implementation overridable

Other implementations of dirstate will want to replace the class used for the
dirstate map.  Allow this to happen by making the class an attribute of the
dirstate.

Differential Revision: https://phab.mercurial-scm.org/D1347

mercurial/dirstate.py

@@ -80,6 +80,7 b' class dirstate(object):'
         self._plchangecallbacks = {}
         self._origpl = None
         self._updatedfiles = set()
+        self._mapcls = dirstatemap
 
     @contextlib.contextmanager
     def parentchange(self):
@@ -128,7 +129,7 b' class dirstate(object):'
     @propertycache
     def _map(self):
         """Return the dirstate contents (see documentation for dirstatemap)."""
-        self._map = dirstatemap(self._ui, self._opener, self._root)
+        self._map = self._mapcls(self._ui, self._opener, self._root)
         return self._map
 
     @property