title: Jekyll post_url -- what am I missing?
body: <p>I am using Jekyll and Github pages, testing locally using <code>bundle exec jekyll serve</code>. Everything else works fine so far.</p>
  
  <p>My posts are in a <code>/_posts</code> folder off of the root, and are outputted correctly.</p>
  
  <p>I cannot seem to get <code>{% post_url %}</code> working within posts for other internal posts, no matter what I do.</p>
  
  <h2>Example / Question</h2>
  
  <ul>
  <li>In my <code>_posts</code> folder, I have a post with a file name of <code>2011-04-14-free-million-dollar-idea-package-carriers-b2b-success.markdown</code>
  <ul>
  <li>This shows up on my site at <code>[root]/2011/04/free-million-dollar-idea-package-carriers-b2b-success/</code></li>
  </ul></li>
  <li>My config specifies <code>permalink:   /:year/:month/:title</code></li>
  </ul>
  
  <p>I have tried the following references within <code>{% post_url %}</code> from another post, to try to link to the post:</p>
  
  <ul>
  <li>2011-04-14-free-million-dollar-idea-package-carriers-b2b-success
  <ul>
  <li>Based on the file name of the post, which I thought was what I should be referencing</li>
  </ul></li>
  <li>/2011/04/free-million-dollar-idea-package-carriers-b2b-success/
  <ul>
  <li>Based on the URL that the post actually resides on at my web site</li>
  </ul></li>
  <li>2011/04/free-million-dollar-idea-package-carriers-b2b-success/
  <ul>
  <li>Without the leading slash</li>
  </ul></li>
  <li>/2011/04/free-million-dollar-idea-package-carriers-b2b-success
  <ul>
  <li>Without the trailing slash</li>
  </ul></li>
  <li>2011/04/free-million-dollar-idea-package-carriers-b2b-success
  <ul>
  <li>Without either slash</li>
  </ul></li>
  </ul>
  
  <p>Each one yields the error:</p>
  
  <blockquote>
    <p>Could not parse name of post [reference] in tag 'post_url'</p>
  </blockquote>
  
  <p>What am I missing?</p>