Function diff_two_refs 

pub fn diff_two_refs(
    gitdir: &Path,
    mem: &str,
    ref_a: &str,
    ref_b: &str,
    config: &DiffConfig,
) -> Result<Diff, BackendError>

Two-ref structural diff. See module docs for the v1 scope and the known gaps (rename, ripple) that will surface as handover items.