title: what is the usage for "repo sync -d" in Android source repository
body: <p>from repo 's help, I found following information:</p>
  
  <blockquote>
    <p>The -d/--detach option can be used to switch specified projects back
    to the manifest revision. This option is especially helpful if the
    project is currently on a topic branch, but the manifest revision is
    temporarily needed.</p>
  </blockquote>
  
  <p>1 Can someone tell me what the situation to use the -d option? </p>
  
  <p>2 Does it means I can get a working repo exactly the same as remote repository, no matter what topic you are working on.</p>