title: Removing a merged pull request from history
body: <p>A pull request was merged in several days ago and we've decided that the whole thing needs to come out.</p>
  
  <p>A few commits have been put in after this action and I'd like to keep those.</p>
  
  <p>I've included a screenshot of my commit graph below. The commit in question that I'd like to remove is <code>19fdbd1</code>.</p>
  
  <p>I attempted to use <code>git rebase 19fdbd1</code> to do this but it did not have the intended effect as the feature in question was still present. I most likely did this incorrectly though.</p>
  
  <p><img src="https://i.stack.imgur.com/dLQtU.png" alt="example"></p>