Prevent pruning remotes with branches available locally

[sergiou87]

Description

When a pull request from a fork is checked out, a remote prefixed with github-desktop- is added locally for the PR's branch. To prevent a clutter of remotes, the app will periodically prune old remotes from forks that are, apparently, not used anymore: if all PRs from a given remote have been closed, that remote will be removed.

However, it's possible that one of those remotes doesn't have any open PR, but branches from them are still available locally. In that case, the remote should be preserved.

This PR makes sure remotes from forked repos are not removed if a branch is available locally. It also adds some unit tests for the function in charge of deciding which remotes must be pruned.

It's important to know that local branches from merged PRs are also removed if they haven't been checked out in more than 2 weeks.

Release notes

Notes: [Fixed] Remotes referencing forked repos would be removed even when branches from them were available locally

[sergiou87]

The work here should be done, but @niik found an issue with allBranches from GitStore: it doesn't actually have all branches, because getBranches from for-each-ref.ts filters out branches from forked repos:

desktop/app/src/lib/git/for-each-ref.ts

Lines 100 to 106 in 510087a

	if (ref.startsWith(ForksReferencesPrefix)) {
	// hide refs from our known remotes as these are considered plumbing
	// and can add noise to everywhere in the user interface where we
	// display branches as forks will likely contain duplicates of the same
	// ref names
	continue
	}

I'll leave this as a draft until that is fixed, @niik is working on it! ❤️

[niik]

I'll leave this as a draft until that is fixed, @niik is working on it! ❤️

#11267 ✨

[niik]

Just a few small requests here but overall this looks great!

[niik]

✨ Thanks for picking this up!