dirstate: move clear onto dirstatemap class

A future diff will move the lazy loading aspect of dirstate to the dirstatemap
class. This means it requires a slightly different strategy of clearing than
just reinstantiating the object (since just reinstantiating the object will
lazily load the on disk data again later instead of remaining permanently
empty).

So let's give it it's own clear function.

Differential Revision: https://phab.mercurial-scm.org/D1252

mercurial/dirstate.py

@@ -593,8 +593,7 b' class dirstate(object):'
         return path
 
     def clear(self):
-        self._map = dirstatemap(self._ui, self._opener, self._root)
-        self._map.setparents(nullid, nullid)
+        self._map.clear()
         self._lastnormaltime = 0
         self._updatedfiles.clear()
         self._dirty = True
@@ -1210,6 +1209,11 b' class dirstatemap(object):'
         # for consistent view between _pl() and _read() invocations
         self._pendingmode = None
 
+    def clear(self):
+        self._map = {}
+        self.copymap = {}
+        self.setparents(nullid, nullid)
+
     def iteritems(self):
         return self._map.iteritems()