Function add 

pub fn add(
    repo: &Path,
    template: &Path,
    template_start: &str,
    wt: &Path,
    new_branch: Option<&str>,
    commitish: Option<&str>,
) -> Result<()>

Create wt by cloning template, refreshed to template_start and warmed from repo first. new_branch and commitish are git worktree add’s; on error nothing is left behind