I’ve been working on setting up mirrors on github of all our BZR branches. My first efforts that are at a suitable stage to share are mirrors of the Oracle MySQL trees. This is currently a snapshot of MySQL 5.1, 5.5 and 5.6 with all the tags preserved. I’ve managed to get GIT to compact down the repository to a mere 177MB on disk for all the history, which is rather impressive.
Go check it out: https://github.com/percona/mysql
This should be considered experimental and I may end up pushing up something better at some point soon – i.e. don’t rely on being able to merge later update (think rebase rather than merge).