Tarmac does a fresh download of the target *before* checking if anything is ready to land

This is somewhat related to bug #397476

Basically, the code in main() first opens the development focus with:
              trunk = Branch(project.development_focus.branch, create_tree=True)

It *then* does:
        candidates = [entry for entry in trunk.landing_candidates
                        if entry.queue_status == u'Approved' and
                        entry.commit_message]

However, the "create_tree=True" means that it will:
1) delete the existing /tmp/$PROJECT directory
2) re-download the full history of the development focus into /tmp/$PROJECT

For large projects with lots of history, this means you'll spend quite a while downloading the data, just to have it say "nope, I don't need that yet, because I don't have anything to land."