The Bzr repositories for projects use separate directories for each branch.
You can see the branch names in the repository by pointing your web browser at http://bzr.savannah.gnu.org/r/FOO/
For a repository with only a top-level .bzr directory use
bzr branch http://bzr.savannah.gnu.org/r/FOO
For a repository with separate branch directories (e.g trunk, devel, etc) use
bzr branch http://bzr.savannah.gnu.org/r/FOO/<branch>
where <branch> is the name of the branch you want.