brisbane: groupcompress cleanups - part1

[Ian Clatworthy]

John Arbash Meinel wrote:

> I'm a bit concerned that some of this is going to conflict with the
> recent changes Vincent and I did for implementing a pure-python version
> of GroupCompress.

Yep - vila and I chatted on irc about this a few hours ago. :-(

I learnt a lot today about how GroupCompress works so the review
wasn't wasted. But I wish I had started it 24 hours later!

Anyhow, you might want to ping vila on IRC so two of you aren't
both resolving the differences independently.

Ian C.