phases: update the phase set as we go during retract boundary

Apparently iterating over the `changed_revs` dictionary is very expensive.

On mozilla-try-2019-02-18, a perf::unbundle call with a 10 000 changesets
bundle gives give use the following timing.

e57d4b868a3e:     4.6 seconds
ac1c75188440:   102.5 seconds
prev-changeset:  30.0 seconds
this-changeset:   4.6 seconds

So, the performance regression is gone.

Once again: thanks to marvelous Python!

mercurial/phases.py

@@ -869,6 +869,8 b' class phasecache:'
         start = min(new_revs)
         end = len(cl)
         rev_phases = [None] * (end - start)
+
+        this_phase_set = self._phasesets[targetphase]
         for r in range(start, end):
 
             # gather information about the current_rev
@@ -893,6 +895,7 b' class phasecache:'
                     new_roots.add(r)
                 rev_phases[r - start] = targetphase
                 changed_revs[r] = r_phase
+                this_phase_set.add(r)
             elif p_phase is None:
                 rev_phases[r - start] = r_phase
             else:
@@ -903,10 +906,10 b' class phasecache:'
                 if p_phase == targetphase:
                     if p_phase > r_phase:
                         changed_revs[r] = r_phase
+                        this_phase_set.add(r)
                     elif r in currentroots:
                         replaced_roots.add(r)
             sets = self._phasesets
-            sets[targetphase].update(changed_revs)
             if targetphase > draft:
                 for r, old in changed_revs.items():
                     if old > public: