title: repo sync error: .repo/manifests/: contains uncommitted changes
body: <p>After issued <code>repo sync</code>, I got a error message like <code>error: .repo/manifests/: contains uncommitted changes</code>, but <code>git status</code> or <code>git diff</code> showed nothing changed.</p>
  
  <p>What's cause of the problem and why it happened?</p>