getsubset: get the unpacker version from the bundler The current setup requires to pass both a packer and, optionally, the version of the unpacker. This is confusing and error prone as the two value cannot mismatch. Instead, we simply grab the version from the packer. This fixes a bug where requesting a cg2 from 'hg bundle' were reported as changegroup 1. I should have caught that in the initial changeset but I missed it somehow.