Function ensure_worktree 

pub fn ensure_worktree(
    repo_root: &Path,
    state: &Path,
    feature: &str,
    committed: impl Into<CommitPrecondition>,
) -> Result<Worktree, Refusal>

Creates or reuses the worktree for feature.

repo_root is the git working tree of the user’s checkout; committed says which repository-relative spec paths must be committed.