bundle2-push: provide transaction to reply unbundler
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.
This diff adds an experimental config option "bundle2.pushback" which provides
a transaction to the reply unbundler during a push operation. This behavior is
opt-in because of potential security issues: the response can contain any part
type that has a handler defined, allowing the server to make arbitrary changes
to the local repository.