Function ensure_submodule_gitdir_config 

pub fn ensure_submodule_gitdir_config(
    work_tree: &Path,
    git_dir: &Path,
    cfg: &mut ConfigFile,
    submodule_name: &str,
) -> Result<String>

Ensures submodule.<name>.gitdir exists, writing it via compute_default_submodule_gitdir if needed.