bundle2: allow extensions to extend the getbundle request

We want extensions to be able to request extra parts.

mercurial/exchange.py

@@ -578,6 +578,7 b' def _pullbundle2(pullop):'
         kwargs['heads'] = pullop.heads or pullop.rheads
         if pullop.heads is None and list(pullop.common) == [nullid]:
             pullop.repo.ui.status(_("requesting all changes\n"))
+    _pullbundle2extraprepare(pullop, kwargs)
     if kwargs.keys() == ['format']:
         return # nothing to pull
     bundle = pullop.remote.getbundle('pull', **kwargs)
@@ -588,6 +589,10 b' def _pullbundle2(pullop):'
     assert len(op.records['changegroup']) == 1
     pullop.cgresult = op.records['changegroup'][0]['return']
 
+def _pullbundle2extraprepare(pullop, kwargs):
+    """hook function so that extensions can extend the getbundle call"""
+    pass
+
 def _pullchangeset(pullop):
     """pull changeset from unbundle into the local repo"""
     # We delay the open of the transaction as late as possible so we