bazaar 2.0beta format for launchpad release

[Martin Pool]

John (with Andrew) has a patch up for brisbane-core stacking
<https://code.edge.launchpad.net/~jameinel/bzr/1.15-gc-stacking/+merge/6852>
that closes the last currently known bug in this format.

I'd like to rename this format to 2.0beta in the next release.
Ideally we'll do that in such a way that if the format is unchanged in
2.0, we won't require a further upgrade to just call it --format 2.0.

The Launchpad open source release will be before the end of July, and
they plan to make that release in this format.  It will be a pretty
big day, and a big tree, and it'd be good to have it in this new
faster and more compact format.

To get there, we need to make a release for testing soon, I think
perhaps a 1.15.1 when this patch is merged and then I'd like to
dogfood it on Launchpad and Bazaar, and to encourage our active user
community to try it too.  If you find problems, please do file bugs
and target them to 2.0 or 1.15.1.

-- 
Martin <http://launchpad.net/~mbp/>