"Compare" repo link should include the current branch/tag #1197

1. Visit https://github.com/BrowserSync/browser-sync/tree/client-ts
2. Open repo tab "More" -> "Compare"
3. See a plain master -> master comparison: https://github.com/BrowserSync/browser-sync/compare

I'd expect that to include a reference to the current branch/commit/tag, like:

https://github.com/BrowserSync/browser-sync/compare/master...client-ts

Maybe there's a better way, but I'm trying to see what changed between master and whatever branch I'm looking at.

This suggestion is similar to #966, but not just between tags.

The default branch can be determined via fetch, exactly like in #1173 is available via https://github.com/sindresorhus/refined-github/blob/master/source/libs/get-default-branch.js