phases: detect when boundaries has been actually retracted

It is useful to detect noop and avoid expensive operations in this case.

We return the information to inform the caller of a possible update. Top level
function might need to react to the phase update (eg: invalidating some
caches, tracking phase change).

mercurial/phases.py

@@ -357,6 +357,7 b' class phasecache(object):'
 
         repo = repo.unfiltered()
         currentroots = self.phaseroots[targetphase]
+        finalroots = oldroots = set(currentroots)
         newroots = [n for n in nodes
                     if self.phase(repo, repo[n].rev()) < targetphase]
         if newroots:
@@ -376,8 +377,10 b' class phasecache(object):'
             finalroots = set(n for n in currentroots if repo[n].rev() <
                              minnewroot)
             finalroots.update(ctx.node() for ctx in updatedroots)
-
+        if finalroots != oldroots:
             self._updateroots(targetphase, finalroots, tr)
+            return True
+        return False
 
     def filterunknown(self, repo):
         """remove unknown nodes from the phase boundary