bundle2: support the 'targetphase' parameter for the changegroup part

By default unbundled changesets are drafts. We want to reduce the number of
phases changes during unbundling by giving the possibility to the bundle to
indicate the phase of unbundled changesets.

The longer terms goal is to add phase movement tracking in tr.changes and the
'retractboundary' call is making it more complicated than we want.

mercurial/bundle2.py

@@ -1517,7 +1517,8 b' def combinechangegroupresults(op):'
         result = -1 + changedheads
     return result
 
-@parthandler('changegroup', ('version', 'nbchanges', 'treemanifest'))
+@parthandler('changegroup', ('version', 'nbchanges', 'treemanifest',
+                             'targetphase'))
 def handlechangegroup(op, inpart):
     """apply a changegroup part on the repo
 
@@ -1542,8 +1543,12 b' def handlechangegroup(op, inpart):'
         op.repo.requirements.add('treemanifest')
         op.repo._applyopenerreqs()
         op.repo._writerequirements()
+    extrakwargs = {}
+    targetphase = inpart.params.get('targetphase')
+    if targetphase is not None:
+        extrakwargs['targetphase'] = int(targetphase)
     ret = _processchangegroup(op, cg, tr, 'bundle2', 'bundle2',
-                              expectedtotal=nbchangesets)
+                              expectedtotal=nbchangesets, **extrakwargs)
     if op.reply is not None:
         # This is definitely not the final form of this
         # return. But one need to start somewhere.