pull: use the "cg" argument when pulling a bundle2

We use the `cg` argument to disable the generation of a changegroup part. This
is useful to pull information when changesets are already in sync (phases,
obsmarkers).

mercurial/exchange.py

@@ -826,6 +826,7 b' def _pullbundle2(pullop):'
 
     kwargs['common'] = pullop.common
     kwargs['heads'] = pullop.heads or pullop.rheads
+    kwargs['cg'] = pullop.fetch
     if 'b2x:listkeys' in remotecaps:
         kwargs['listkeys'] = ['phase']
     if not pullop.fetch: