bundle2: no longer use 'retractboundary' in updatephases

The new 'phase-heads' forced all added node to secret before advancing the
boundary to work around the fact changesets were added as draft by default.
This is no longer necessary since the changegroup part can now use the
'targetphase' parameter.

Not doing this retract boundary call has a couple of advantages:

* This makes implementing phases change tracking in the transaction much
  simpler since retract boundary can become a rare case.

* Bundling secret changesets is not the norm. Exchange never does that and
  even for strip, the use-case is not common.Skipping the retract boundary
  will avoid useless work here.

* Sending phase update on push can be simplified since we can rely on the
  behavior of 'cg.apply' for most of it.
  This means less phases update send for example.

* We no longer needs to track and use the addednodes during unbundling. This
  make it possible to have multiple 'changegroup' and 'phase-heads' parts in the
  same bundle without them interfering with each others.

The new part has not been part of any release yet so we do not offer backward
compatibility yet. It is important to update this semantic before the 4.3
freeze happens.

mercurial/phases.py

@@ -448,10 +448,6 b' def subsetphaseheads(repo, subset):'
 
 def updatephases(repo, tr, headsbyphase, addednodes):
     """Updates the repo with the given phase heads"""
-    # First make all the added revisions secret because changegroup.apply()
-    # currently sets the phase to draft.
-    retractboundary(repo, tr, secret, addednodes)
-
     # Now advance phase boundaries of all but secret phase
     for phase in allphases[:-1]:
         advanceboundary(repo, tr, phase, headsbyphase[phase])