phase: also copy phase's sets when copying phase cache

We forgot to add such copy when we added the attributes.

mercurial/phases.py

@@ -167,6 +167,7 b' class phasecache(object):'
         ph.dirty = self.dirty
         ph.opener = self.opener
         ph._phaserevs = self._phaserevs
+        ph._phasesets = self._phasesets
         return ph
 
     def replace(self, phcache):