pull: add a set of steps that remain to be done during the pull

With bundle2 we'll slowly move current steps into a single bundle2 building and
call (changegroup, phases, obsmarkers, bookmarks). But we need to keep the
old methods around for servers that do not support bundle2. So we introduce
this set of steps that remains to be done.

mercurial/exchange.py

@@ -403,6 +403,8 b' class pulloperation(object):'
         self.fetch = None
         # result of changegroup pulling (used as returng code by pull)
         self.cgresult = None
+        # list of step remaining todo (related to future bundle2 usage)
+        self.todosteps = set(['changegroup', 'phases', 'obsmarkers'])
 
     @util.propertycache
     def pulledsubset(self):
@@ -451,9 +453,12 b' def pull(repo, remote, heads=None, force'
     lock = pullop.repo.lock()
     try:
         _pulldiscovery(pullop)
-        _pullchangeset(pullop)
-        _pullphase(pullop)
-        _pullobsolete(pullop)
+        if 'changegroup' in pullop.todosteps:
+            _pullchangeset(pullop)
+        if 'phases' in pullop.todosteps:
+            _pullphase(pullop)
+        if 'obsmarkers' in pullop.todosteps:
+            _pullobsolete(pullop)
         pullop.closetransaction()
     finally:
         pullop.releasetransaction()
@@ -477,6 +482,7 b' def _pullchangeset(pullop):'
     # We delay the open of the transaction as late as possible so we
     # don't open transaction for nothing or you break future useful
     # rollback call
+    pullop.todosteps.remove('changegroup')
     if not pullop.fetch:
             pullop.repo.ui.status(_("no changes found\n"))
             pullop.cgresult = 0
@@ -505,6 +511,7 b' def _pullchangeset(pullop):'
 
 def _pullphase(pullop):
     # Get remote phases data from remote
+    pullop.todosteps.remove('phases')
     remotephases = pullop.remote.listkeys('phases')
     publishing = bool(remotephases.get('publishing', False))
     if remotephases and not publishing:
@@ -529,6 +536,7 b' def _pullobsolete(pullop):'
     a new transaction have been created (when applicable).
 
     Exists mostly to allow overriding for experimentation purpose"""
+    pullop.todosteps.remove('obsmarkers')
     tr = None
     if obsolete._enabled:
         pullop.repo.ui.debug('fetching remote obsolete markers\n')