Function worktree_add 

pub fn worktree_add(
    repo: &Path,
    path: &Path,
    branch: &str,
    base: Option<&str>,
    new_branch: bool,
    lock: bool,
    lock_reason: Option<&str>,
) -> Result<(), WorktreeError>

Run git worktree add with the appropriate flags. Returns Ok(()) on success.