push: extract future heads computation into pushop

Bundle2 will allow pushing all different parts of the push in a single bundle.
This mean that the discovery for each part needs to be done before trying to
push. Currently we may have different behaviors for phases and obsolescence markers
when the push of changesets fails. For example, information may still be
exchanged for a part of the history where changesets are common but where
phases mismatch. So the preparation of the push need to determine what
information need to be pushed in both situations. And it needs a different set of
heads for this. Therefore we are moving heads computation within pushop for easy
access by all parties. We start with the simplest set of heads.

mercurial/exchange.py

@@ -80,6 +80,11 b' class pushoperation(object):'
         # set of all heads common after changeset bundle push
         self.commonheads = None
 
+    @util.propertycache
+    def futureheads(self):
+        """future remote heads if the changeset push succeeds"""
+        return self.outgoing.missingheads
+
 def push(repo, remote, force=False, revs=None, newbranch=False):
     '''Push outgoing changesets (limited by revs) from a local
     repository to remote. Return an integer:
@@ -310,8 +315,7 b' def _pushchangeset(pushop):'
 def _pushcomputecommonheads(pushop):
     unfi = pushop.repo.unfiltered()
     if pushop.ret:
-        # push succeed, synchronize target of the push
-        cheads = pushop.outgoing.missingheads
+        cheads = pushop.futureheads
     elif pushop.revs is None:
         # All out push fails. synchronize all common
         cheads = pushop.outgoing.commonheads