phases: fast path retract of public phase

There are no boundary to retract, so lets do nothing.

mercurial/phases.py

@@ -674,6 +674,8 b' class phasecache:'
         repo.invalidatevolatilesets()
 
     def _retractboundary(self, repo, tr, targetphase, nodes=None, revs=None):
+        if targetphase == public:
+            return False
         # Be careful to preserve shallow-copied values: do not update
         # phaseroots values, replace them.
         if revs is None: