changegroup: use tr.hookargs when calling pretxnchangegroup hooks So that other parties using the transaction can put information in our hook calls.

addchangegroup: register data in tr.hookargs We are registering data related to the process into the transaction hook data. This lets other parties using the same transaction get informed of the addchangegroup result.

transaction: add a notion of hook arguments It is now possible to register parameters to be used when invoking hooks in this transaction. This will cope with the fact that bundle2 adds multiple data types in a single transaction. Do not expect any wide and consistent usages of this in the next release. This will be used by bundle2 experiments first. It will be made better for the release after.

bundle2: allow extensions to plug into the push process Extensions are offered functions to add parts and process their results.

bundle2: require both client and server to opt in Even if the server is bundle2-enabled, the client now has to opt-in in the config too.

bundle2: move bundle2 config option to section "experimental" We highlight the fact that this is experimental by moving it to an "experimental" section, and we match the config name with the server capability name `bundle2-exp`.