title: Selecting other branch instead of master as a source repository on AWS CodeBuild
body: <p>How to specify different branch instead of <code>master</code> branch on AWS code deploy while using <code>Github</code> as source provider ? I see there is no option to select in the console to select branch(may be I missed).</p>
  
  <p>I tried to checkout to different branch while we are in the <code>pre_build</code> phase, but it failed in the <strong>Download Source</strong> phase itself as the <code>master</code> branch doesn't have YAML file.</p>
  
  <pre><code>version: 0.1
  phases:
    install:
      commands:
        - apt-get update -y
    pre_build:
      commands:
        - git checkout testbranch
  </code></pre>