Userscript that adds some UI to see the diff between 2 commits on GitHub or BitBucket
To use it, first install an extension to handle userscripts (like Greasemonkey for Firefox or Tampermonkey for Chrome) and then install the script here
It should appear as a "D" button next to the links to commits, for example in the "Commits" tab of a pull request. Click on the button next to the first commit and then on the one for the last commit to see the comparison between those 2 (note that bitbucket sorts them newest to oldest while github is oldest to newest, the buttons should be pressed oldest first and newest later)