phase: put retractboundary out of the loop in advanceboundary

It seems that we were calling retractboundary for each phases to process.
Putting the retractboundary out of the loop reduce the number of calls,
helping tracking the phases changes.

mercurial/phases.py

@@ -301,9 +301,9 b' class phasecache(object):'
                 self._updateroots(phase, roots, tr)
                 # some roots may need to be declared for lower phases
                 delroots.extend(olds - roots)
-            # declare deleted root in the target phase
-            if targetphase != 0:
-                self.retractboundary(repo, tr, targetphase, delroots)
+        # declare deleted root in the target phase
+        if targetphase != 0:
+            self.retractboundary(repo, tr, targetphase, delroots)
         repo.invalidatevolatilesets()
 
     def retractboundary(self, repo, tr, targetphase, nodes):