push: move `commonheads` into the push object

The phase synchronisation start by computing the new set of common head between
local and remote and then do the phase synchronisation on this set. This new
common set logic will eventually be used by the obsolescence markers exchange.
So we are going to split the long phase synchronisation in two.

mercurial/exchange.py

@@ -48,6 +48,8 b' class pushoperation(object):'
         self.remoteheads = None
         # testable as a boolean indicating if any nodes are missing locally.
         self.incoming = None
+        # set of all heads common after changeset bundle push
+        self.commonheads = None
 
 def push(repo, remote, force=False, revs=None, newbranch=False):
     '''Push outgoing changesets (limited by revs) from a local
@@ -238,6 +240,7 b' def _pushsyncphase(pushop):'
                          pushop.outgoing.commonheads,
                          pushop.outgoing.missing)
         cheads.extend(c.node() for c in revset)
+    pushop.commonheads = cheads
     # even when we don't push, exchanging phase data is useful
     remotephases = pushop.remote.listkeys('phases')
     if (pushop.ui.configbool('ui', '_usedassubrepo', False)