exchange: have `pull` return the pulloperation object

We mimic what was done for `push` for similar reason. We are about to drop
`localrepo.pull` (for consistency with dropping `localrepo.push` and we better
have an API as extensible as `push` is.

Find explanations about localrepo.push removal in 4d52e6eb98ea.

mercurial/exchange.py

@@ -866,7 +866,7 b' def pull(repo, remote, heads=None, force'
         pullop.releasetransaction()
         lock.release()
 
-    return pullop.cgresult
+    return pullop
 
 def _pulldiscovery(pullop):
     """discovery phase for the pull

mercurial/localrepo.py

@@ -1575,7 +1575,7 b' class localrepository(object):'
         return r
 
     def pull(self, remote, heads=None, force=False, **kwargs):
-        return exchange.pull(self, remote, heads, force, **kwargs)
+        return exchange.pull(self, remote, heads, force, **kwargs).cgresult
 
     def checkpush(self, pushop):
         """Extensions can override this function if additional checks have