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`.

bundle2: move all parts into a `bx2` namespace All currently core parts are moved to a `bx2` namespace (for "bundle 2 experimental"). This should avoid conflicts between the final stable format and the one about to be released.

bundle2: rename server capability to bundle2-exp For the same reason, we advertise this bundle2 implementation and format as experimental. This will leave room for field testing in 3.0 but won't conflict with a stable implementation in 3.1.

bundle2: use HG2X in the header The current implementation of bundle2 is still very experimental and the 3.0 freeze is yesterday. The current bundle2 format has never been field-tested, so we rename the header to HG2X. This leaves the HG20 header available for real usage as a stable format in Mercurial 3.1. We won't guarantee that future mercurial versions will keep supporting this `HG2X` format.

bundle2: transmit capabilities to getbundle during pull Bundle2 capabilities of the client are sent to the server in the bundlecaps argument of `getbundle`.

bundle2: include client capabilities in the pushed bundle The necessary data is now included in the `replycaps` part.

bundle2: advertise bundle2 caps in server capabilities We can now retrieve them from the server during push. The capabilities are encoded the same way as in `replycaps` part (with an extra layer of urlquoting to escape separators).