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.