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.