title: git checkout remote reference
body: <p>I have a list of pull requests on github. I can fetch the pull requests like this:</p>
  
  <pre><code>git fetch origin +refs/pull/*:refs/remotes/origin/pr/*
  </code></pre>
  
  <p>I get output like this:</p>
  
  <pre><code>* [new ref]         refs/pull/1/head -&gt; origin/pr/1/head
  * [new ref]         refs/pull/1/merge -&gt; origin/pr/1/merge
  * [new ref]         refs/pull/10/head -&gt; origin/pr/10/head
  * [new ref]         refs/pull/10/merge -&gt; origin/pr/10/merge
  * [new ref]         refs/pull/11/head -&gt; origin/pr/11/head
  * [new ref]         refs/pull/11/merge -&gt; origin/pr/11/merge
  </code></pre>
  
  <p>Now I want to checkout one of those refs. Nothing I try seems to work:</p>
  
  <pre><code>$ git checkout refs/pull/1/head
  error: pathspec 'refs/pull/1/head' did not match any file(s) known to git.
  </code></pre>
  
  <p>Or:</p>
  
  <pre><code>git checkout origin/pr/1/head
  error: pathspec 'origin/pr/1/head' did not match any file(s) known to git.
  </code></pre>
  
  <p>How can I checkout this reference?</p>