bundle2: allow extensions to extend the getbundle request We want extensions to be able to request extra parts.

bundle2: add a way to add parts during a `getbundle` request We add a hook function to let extensions add parts to the bundle.

getbundle: pass arbitrary arguments all along the call chain The wireprotocol command accepts arbitrary arguments, we are now passing such arguments down the call chain.

bundle2: call a hook after the transaction is closed We call a dedicated hook right after closing the transaction. This will let people react to the transaction with all the information in hand. This hook is experimental and will not survive in future versions.

bundle2: call a hook prior to closing the transaction We call a dedicated hook right before closing the transaction. This will let people abort unbundling with all the information in hand. This hook is experimental and will not survive in future versions.

bundle2: inform transaction that we are in a bundle2 unbundle That way, hooks called during the unbundle process are aware that a bigger picture is going on.

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

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.