Create a new bookmark with the following code (Github doesn't allow making a draggable bookmarklet) or paste the IIFE (top of the JS file below) into the console on a Github compare page.
javascript:(function()%7B(()%20%3D%3E%20%7Bconst%20url%20%3D%20document.location.href%3Bconst%20targetBasename%20%3D%20url.split('%2F').slice(-1)%5B0%5D.split('...').reverse().join('...')%3Bconst%20lastSlash%20%3D%20url.lastIndexOf('%2F')%3Bconst%20targetURL%20%3D%20url.substring(0%2C%20lastSlash%20%2B%201)%20%2B%20targetBasename%3Bconsole.log('Reversed%20branch%20compare%20URL.%20Navigating%20to'%2C%20targetURL)%3Bdocument.location.href%20%3D%20targetURL%3B%7D)()%7D)()
I prefer to put the bookmarklet in a folder somewhere, hit ⌘+L, type reverse
and tap down to the bookmarklet like so: