title: magit for `pull --rebase upstream` (not origin)
body: <p>Let's say we have an <code>upstream</code> git repository which is the official home of a project, and <code>origin</code> is our own remote git repository (e.g. a github fork).</p>
  
  <p>On the command line, I would rebase to the master before issuing a PR with</p>
  
  <pre><code>git pull --rebase upstream master
  </code></pre>
  
  <p>but how do I do this in magit? The <code>F</code> command allows a <code>--rebase</code> but that's always against my <code>origin</code> as far as I can tell.</p>
  
  <p>Ideally, I'd like to do this with a single command under the <code>F</code> menu as it is a very common operation for me.</p>