changegroup: emit delta group close chunk outside of deltagroup()
I want to make deltagroup() emit data structures rather than
serialized deltas. Upcoming commits will demonstrate why.
Differential Revision: https://phab.mercurial-scm.org/D4207
changegroup: extract cgpacker.group() to standalone function
It doesn't need to be part of the packer class.
Differential Revision: https://phab.mercurial-scm.org/D4206