title: What does it mean 'upstream' in the manifest file, repo?
body: <p>I found the 'upstream' attribute in manifest file, repo.
  I'm not sure what it means during 'repo sync' and 'repo sync -c'
  And what's the difference between projectA and projectB below example.</p>
  
  <p>During 'repo sync', I think it will clone projectA with commit '12345',
  In projectB case, it will also clone projectB with commit 'abcde'.
  Why 'upstream' is needed?</p>
  
  <p>Please could you explain what it means?</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;manifest&gt;
      &lt;project name="projectA" path="project/A" remote="aosp" revision="12345" upstream="projectA/develop"/&gt;
      &lt;project name="projectB" path="project/B" remote="aosp" revision="abcde" /&gt;
  &lt;/manifest&gt;
  </code></pre>