Install hook fails for bzr projects

We've been trying unsuccessfully to deploy ubucon.org with Juju, which worked fine for a few months until recently. We've tracked it to be a failed install hook that fails at doing a bzr checkout:

http://bazaar.launchpad.net/~charm-helpers/charm-helpers/devel/view/head:/charmhelpers/fetch/bzrurl.py#L62

You can see it in the code that that can never work:

1. Create directory
2. If directory exists, do a `bzr pull`
3. Of course the directory exists, but it's not a bzr branch, so the `pull` fails

I've noticed this stopped working after the charm helpers migration to Python 3.0, where the original bzr API code was replaced by process calls. However, in doing the replacement, it seems the tests were not updated, as they seem to pass even though the code does not work.

Steps to reproduce:

    cd $HOME/charms/layers
    bzr branch lp:~ubucon-site-developers/ubucon-site/ubucon-layer ubucon
    cd ubucon

    charm build
    juju deploy postgresql
    juju deploy $HOME/charms/builds/ubucon
    # Deployment fails during the deploy step, as the
    # install hook cannot do a bzr checkout of the site's code

    juju add-relation postgresql:db ubucon
    juju expose ubucon