bundle2.processbundle: let callers request default behavior

This patch series is intended to allow bundle2 push reply part handlers to
make changes to the local repository; it has been developed in parallel with
an extension that allows the server to rebase incoming changesets while applying
them.

The default transaction getter for processbundle is a private function that
raises an exception; this diff lets calling code pass None as the transaction
getter to explicitly request this default behavior.

The next diff will check a config option to determine whether to provide a
transaction to the reply bundle processor.  If one shouldn't be provided, the
code needs a way to specify that the default behavior should be used.

mercurial/bundle2.py

@@ -277,7 +277,7 b' def _notransaction():'
     to be created"""
     raise TransactionUnavailable()
 
-def processbundle(repo, unbundler, transactiongetter=_notransaction):
+def processbundle(repo, unbundler, transactiongetter=None):
     """This function process a bundle, apply effect to/from a repo
 
     It iterates over each part then searches for and uses the proper handling
@@ -288,6 +288,8 b' def processbundle(repo, unbundler, trans'
 
     Unknown Mandatory part will abort the process.
     """
+    if transactiongetter is None:
+        transactiongetter = _notransaction
     op = bundleoperation(repo, transactiongetter)
     # todo:
     # - replace this is a init function soon.