Add compare link on force push events

What changed in a force push? We should add a link to a compare page between the 2 commits

<img width="561" alt="Screen Shot" src="https://user-images.githubusercontent.com/1402241/103255561-5dcea580-494f-11eb-8d28-1cdf4a83b1a5.png">


Test URL: https://github.com/sindresorhus/refined-github/pull/3377#event-3578643888