Add the tag button provided by "branch-buttons" to the compare page #2018

Example URL: https://github.com/sindresorhus/refined-github/compare/19.5.6.1914...master

Often when comparing branches I want to make the base the latest tag. Using the dropdown - you cannot select tags, you need to type them. I find myself often opening the home page of the repo in a new tab and looking at the button provided by the "branch-buttons" for the latest tag.

It'd be awesome if the button would also set the "base" dropdown to the tag, but maybe that's a little too opinionated?

This is very similar to #1197 - I'm not sure if it's sufficiently different to have a separate issue 😅