pull: move return code in the pull operation object

With bundle2 we'll have multiple code path susceptible to be responsible for
adding changeset during the pull. We move it to the pull operation to simplify
this the coming logic. The one doing the adding the changegroup will set the
value.

mercurial/exchange.py

@@ -401,6 +401,8 b' class pulloperation(object):'
         self.rheads = None
         # list of missing changeset to fetch remotly
         self.fetch = None
+        # result of changegroup pulling (used as returng code by pull)
+        self.cgresult = None
 
     @util.propertycache
     def pulledsubset(self):
@@ -455,9 +457,9 b' def pull(repo, remote, heads=None, force'
         pullop.common, pullop.fetch, pullop.rheads = tmp
         if not pullop.fetch:
             pullop.repo.ui.status(_("no changes found\n"))
-            result = 0
+            pullop.cgresult = 0
         else:
-            result = _pullchangeset(pullop)
+            pullop.cgresult = _pullchangeset(pullop)
 
         _pullphase(pullop)
         _pullobsolete(pullop)
@@ -466,7 +468,7 b' def pull(repo, remote, heads=None, force'
         pullop.releasetransaction()
         lock.release()
 
-    return result
+    return pullop.cgresult
 
 def _pullchangeset(pullop):
     """pull changeset from unbundle into the local repo"""