title: Checkout branch on different remote
body: <p>I have a repo that has another remote <code>upstream</code> besides <code>origin</code>. I can do <code>git checkout origin/master</code>, but when I run <code>git checkout upstream/master</code>, I get:</p>
  
  <pre><code>error: pathspec 'upstream/master' did not match any file(s) known to git.
  </code></pre>
  
  <p>This does not work either:</p>
  
  <pre><code>$ git fetch upstream
  From https://github.com/getsentry/sentry
   * branch            HEAD       -&gt; FETCH_HEAD
  $ git co -b asdf --track upstream/master
  fatal: Cannot update paths and switch to branch 'asdf' at the same time.
  Did you intend to checkout 'upstream/master' which can not be resolved as commit?
  </code></pre>
  
  <p>How to check out branches on <code>upstream</code> remote as I do on <code>origin</code> remote? My git version is 2.1.2.</p>