title: repo tool unable to pull based on tags
body: <p>Im using repo tool to manage multiple repositories.</p>
  
  <p>Here is my sample manifest file and I have given a tag name in my revision - 'firsttag'.</p>
  
  <p>When I do a repo sync it says - </p>
  
  <pre><code>Fetching project repo1
  fatal: Couldn't find remote ref refs/heads/firsttag
  &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  </code></pre>
  
  <p>xml file
      
        </p>
  
  <pre><code>  &lt;!-- &lt;default revision="refs/heads/master" --&gt;
    &lt;default revision='firsttag'
             remote="github"
             sync-j="4" /&gt;
  
    &lt;project name="repo1" remote="github" path="repo1"/&gt;
    &lt;project name="repo2" remote="github" path="repo2"/&gt;
  
  &lt;/manifest&gt;
  </code></pre>