context: return dirstate parents in workingctx.ancestors()

workingctx.ancestors() was not returning the dirstate parents as part of the
result set. The only place this function is used is for copy detection when
committing a file, and that code already checks the parents manually, so this
change has no affect at the moment.

I found it while playing around with changing how copy detection works.

mercurial/context.py

@@ -1165,6 +1165,8 b' class committablectx(basectx):'
         return sorted(self._repo.dirstate.matches(match))
 
     def ancestors(self):
+        for p in self._parents:
+            yield p
         for a in self._repo.changelog.ancestors(
             [p.rev() for p in self._parents]):
             yield changectx(self._repo, a)