This PR improves the existing support to linked-worktrees, making it more reliable while also adding worktree management capabilities.

Opening linked-worktree repositories

PlainOpen():

Opening linked-worktree repositories using PlainOpen requires EnableDotGitCommonDir to be set to true:

PlainOpenWithOptions("/repo", &PlainOpenOptions{EnableDotGitCommonDir: true})

This PR does not change this. Before the V6 release, we may need to consider dropping off the use of EnableDotGitCommonDir and having that working behind the scenes as ideally users would not need to know they are dealing with a linked-worktree, especially when using PlainOpen.

Open()

Using Open requires the storer FS to be a wrapping between both gitDir and commonDir via dotgit.NewRepositoryFilesystem(gitDirFS, commonDirFS).
A new Worktree.Open helper was introduced in x/plumbing/worktree to abstract that logic, so that it automatically does that when it detects the repository being opened is a linked-worktree. Otherwise it falls back to the existing git.Open() logic.

go-git specific features

A new Worktree.Init has been introduced so that users can connect a new filesystem to an existing worktree metadata.
This is useful when the main repository is on disk, and the worktree is an empty in-memory filesystem - or vice-versa.
For example:

	// Setup repository storage on osfs pointing to existing dotgit which contains the feature-x worktree metadata.
	storerFS := osfs.New("/path/to/repo/.git")
	storer := filesystem.NewStorage(storerFS, cache.NewObjectLRUDefault())
	w, err := xworktree.New(storer)
	if err != nil {
		panic(err)
	}

	// Now connect a fresh memfs filesystem to the existing worktree metadata.
	// This is useful when you want the worktree on a different filesystem
	// or filesystem type.
	freshWtFS := memfs.New()
	err = w.Init(freshWtFS, "feature-x")
	if err != nil {
		panic(err)
	}

	// Open the worktree with the new filesystem.
	r, err := w.Open(freshWtFS)
	if err != nil {
		panic(err)
	}

	// The worktree is now connected and can be used.
	_, _ = r.Head()

Making changes to linked-worktrees

The RepositoryFilesystem had a bug that at times would return the incorrect FS during writing changes to a linked-worktree. This has been fixed as part of #1801 (thanks @Soph), I pulled that change into this PR as it is needed for this feature to be fully operational.

Worktree Management

Three core features from upstream are supported: add, remove and list. Note they may not be exposing exactly the same values as what the upstream CLI does, however, it provides enough information so that users can attain the same results - as per go-git/cli PR which implements those subcommand in a go-git native way.

Closes: #41 #1580
Supersedes: #1801 #1675 #396
Relates to: #285 #394

⚠️ The new code is being exposed under x, to avoid a potentially unstable API being a blocker for cutting the V6 release.