add dirstate._dirtypl variable

Theoretically, it's possible to forget modified dirstate
parents by doing:

dirstate.invalidate()
dirstate.setparents(p1, p2)
dirstate._map

The final access to _map should call _read(), which will
unconditionally overwrite dirstate._pl.

This doesn't actually happen right now because invalidate
accidentally ends up rebuilding dirstate._map.

mercurial/dirstate.py

@@ -21,6 +21,7 b' class dirstate(object):'
         self._opener = opener
         self._root = root
         self._dirty = 0
+        self._dirtypl = 0
         self._ui = ui
 
     def __getattr__(self, name):
@@ -114,6 +115,7 b' class dirstate(object):'
 
     def setparents(self, p1, p2=nullid):
         self.markdirty()
+        self._dirtypl = 1
         self._pl = p1, p2
 
     def setbranch(self, branch):
@@ -126,6 +128,7 b' class dirstate(object):'
     def _read(self):
         self._map = {}
         self._copymap = {}
+        if not self._dirtypl:
         self._pl = [nullid, nullid]
         try:
             st = self._opener("dirstate").read()
@@ -135,6 +138,7 b' class dirstate(object):'
         if not st:
             return
 
+        if not self._dirtypl:
         self._pl = [st[:20], st[20: 40]]
 
         # deref fields so they will be local in loop
@@ -262,6 +266,7 b' class dirstate(object):'
         st.write(cs.getvalue())
         st.rename()
         self._dirty = 0
+        self._dirtypl = 0
 
     def filterfiles(self, files):
         ret = {}