Collapse all files in github's PR while viewing changes tab/section in PR.

gistfile1.txt

// JUST run below code in your browser's console, in the browser's tab where PR is open.
var x = document.getElementsByClassName("js-details-container");
  var i;
  for (i = 0; i < x.length; i++) {
    x[i].className = x[i].className.replace(/\bDetails--on\b/g, "");
    x[i].className = x[i].className.replace(/\bopen\b/g, "");
  }