bundle2: introduce an "applybundle" function

There is a case where the intent is clear and the transaction is not optional. We
want to be able to alter that transaction in a wide and easy way. We cannot get
a unified '.apply(repo)' method for bundle1 and bundle2 yet because the api are
still a bit too far apart. But this is a good step forward to get the rc out.

mercurial/bundle2.py

@@ -302,6 +302,10 b' def _notransaction():'
     to be created"""
     raise TransactionUnavailable()
 
+def applybundle(repo, unbundler, tr, op=None):
+    # transform me into unbundler.apply() as soon as the freeze is lifted
+    return processbundle(repo, unbundler, lambda: tr, op=op)
+
 def processbundle(repo, unbundler, transactiongetter=None, op=None):
     """This function process a bundle, apply effect to/from a repo