Function add_or_plain 

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

add_with, or git worktree add with the same wt, new_branch and commitish when cloning fails. A failed clone leaves nothing behind, so the plain add starts clean