title: Amend a commit that wasn't the previous commit
body: <p>Frequently I'll have a workflow like the following:</p>
  
  <ol>
  <li>Commit changes to a group of files</li>
  <li>Commit changes to a different group of files</li>
  <li>Realize I missed some changes that belong in the first commit</li>
  <li>Curse</li>
  </ol>
  
  <p>I can't make use of <code>git commit --amend</code> because it's not the most recent commit that I need to change. What's the best way to add changes to the first commit <strong>without touching the second one?</strong></p>