title: How can I verify git argument @{u} is working correctly?
body: <p>I'm having problems with <code>flutter upgrade</code>, which uses:</p>
  <pre><code>D:\personal-dev\flutter&gt;git rev-parse --verify @{u}
  fatal: Needed a single revision
  </code></pre>
  <p>However, this works:</p>
  <pre><code>D:\personal-dev\flutter&gt;git rev-parse --verify origin/stable
  78910062997c3a836feee883712c241a5fd22983
  </code></pre>
  <p>Any ideas on how I could produce more diagnostic info?</p>