patch-viewer/ext/patch-viewer.js

var ELEMENT_ID = 'viewPatchButton';

var parent = document.querySelector('.commit');
var sibling = parent.querySelector('.btn');
var el = document.createElement('a');

if (!document.querySelector('#' + ELEMENT_ID)) {
  el.href = window.location.href + '.patch';
  el.classList = 'btn btn-outline float-right';
  el.innerText = 'View as Patch';
  el.style.marginLeft = '1em';
  el.id = ELEMENT_ID;

  parent.insertBefore(el, sibling);
}