push: elevate phase transaction to cover entire operation 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. Most pushes already open a transaction in order to sync phase information. This diff replaces that transaction with one that spans the entire push operation. This transaction will be used in a later patch to guard repository changes made during the reply handler.