worktree: implement `git_worktree_open_from_repository`

worktree: implement `git_worktree_open_from_repository`

While we already provide functionality to look up a worktree from a
repository, we cannot do so the other way round. That is given a
repository, we want to look up its worktree if it actually exists.
Getting the worktree of a repository is useful when we want to get
certain meta information like the parent's location, getting the locked
status, etc.