title: How to refer to a previous commit in git commit message
body: <p>Is there a convention for referring to a previous commit in a git commit message?</p>
  
  <p>Example commit message:</p>
  
  <pre><code>Fixed bug such and such introduced in a1b2c3e4
  </code></pre>
  
  <p>In particular, is there a convention that github.com will understand, and convert to a link?</p>