changegroup: introduce "raw" versions of some commands The commands getchangegroup, getlocalchangegroup and getsubset now each have a version ending in -raw. The raw versions return the chunk generator from the changegroup packer directly, without wrapping it in a chunkbuffer and unpacker. This avoids extra chunkbuffers in the bundle2 code path. Also, the raw versions can be extended to support alternative packers in the future, to be used from bundle2.