phases: fast path public phase advance when everything is public

Everything is already public, so we have nothing to do here.

mercurial/phases.py

@@ -597,6 +597,8 b' class phasecache:'
 
         Returns a set of revs whose phase is changed or should be changed
         """
+        if targetphase == public and not self.hasnonpublicphases(repo):
+            return set()
         # Be careful to preserve shallow-copied values: do not update
         # phaseroots values, replace them.
         if revs is None: