title: How to completely remove a merged pull request?
body: <p>I merged a pull request on GitHub. However, it was a mistake, and I want to undo the merge, its commits, and most importantly, delete all the files introduced by these several commits from the history.</p>
  
  <p>I reverted the commit with another one and it works, but the files are still in the commit history (they're big) and it would be nice if the commits themselves disappeared.</p>
  
  <p>I've tried to filter-branch removing the files, but it doesn't seem to be shrinking the repository size (and the commits are still there).</p>
  
  <p>What should I do?</p>