title: git repo sync with another group
body: <p>Is it possible call  <strong>repo sync</strong> with specific another <strong>gruop</strong></p>
  
  <p>eg:</p>
  
  <pre><code>repo init -u git@$1.1.1.1/xyz.git -g A
  repo sync
  </code></pre>
  
  <p>after synced, I will like to add another group <strong>B</strong> in <strong>manifest.xml</strong>
  then I would like to test the group <strong>B</strong>, such as</p>
  
  <pre><code>repo sync -g B   &lt;== repo sync seems not not support option **B**
  </code></pre>