pull: extract transaction logic into separate object 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. Aside from the transaction logic, the pulloperation class is used primarily as a logic-free data structure for storing state information. This diff extracts the transaction logic into its own class that can be shared with push operations.