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.