changegroup: introduce cg2packer/unpacker cg2 supports generaldelta in changegroups, to be used in bundle2. Since generaldelta is handled directly in cg2, reordering is switched off by default.

push: send highest changegroup format supported by both side When using bundle2, we find the common subset of supported changegroup-packers and we pick the max of them. This allow to use generaldelta aware changegroups through bundle2.