`default-branch-button` on forks: add secondary button to see folder on source repo

1. Visit https://github.com/fregante/refined-github/blob/incremental-tag-changelog-link/source/background.ts
2. There should be a << link to https://github.com/sindresorhus/refined-github/blob/master/source/background.ts

Example: (ignore the arrow)

How:

1. Edit the get-default-branch function to accept a user/repo string (and it should only use parseBranchFromDom if the string matches the current page)
2. Add the button on forks using the CSS code here: https://github.com/sindresorhus/refined-github/pull/1132

More issues semi-related to this one: https://github.com/sindresorhus/refined-github/pull/1115 https://github.com/sindresorhus/refined-github/pull/1168 https://github.com/sindresorhus/refined-github/pull/1557

[yakov116]

I just realized that Unrelated Feature 😄 it exactly this!

Can I have a go for it?

Played around:

Version 1

Version 2

Version 3

[fregante]

If we change the pathname we mess up this line

export function getForkedRepo(): string | undefined {
- 	return select<HTMLAnchorElement>('.fork-flag a')?.pathname.slice(1);
+ 	return select<HTMLAnchorElement>('.fork-flag a')?.pathname.slice(1).split('/', 2).join('/');
}