That’s how you specify branches in puppetlabs’ spec_helper fixtures.yml:
fixtures: forge_modules: stdlib: repo: "puppetlabs/stdlib" ref: "4.6.0" symlinks: mapr_helpers: "#{source_dir}" repositories: mapr: repo: "http://myurl/puppet-mapr.git" # this is a branch. stolen from: don't remember. ref: 'remotes/origin/ab/devel'