phasees: properly shallow caopy the phase sets dictionary

We are about to increments the set more incrementally in some case, so we need
to make a proper shallow copy of it.

mercurial/phases.py

@@ -471,7 +471,10 b' class phasecache:'
         ph._phaseroots = self._phaseroots.copy()
         ph.dirty = self.dirty
         ph._loadedrevslen = self._loadedrevslen
-        ph._phasesets = self._phasesets
+        if self._phasesets is None:
+            ph._phasesets = None
+        else:
+            ph._phasesets = self._phasesets.copy()
         return ph
 
     def replace(self, phcache):