title: Gitlab-CI: checkout other branch than the one pushed to
body: <p>I have a setup where I have two branches in a repository. </p>
  
  <p><code>dev_software</code> and
  <code>dev_content</code></p>
  
  <p><code>dev_content</code> is setup to hold a git submodule of <code>dev_content</code>.</p>
  
  <p>When content developers push to <code>dev_content</code> I need to trigger the same build process as when software developers push to <code>dev_software</code>. Therefore I need to checkout <code>dev_software</code> in CI on every push.</p>
  
  <p>Howto do that?</p>