Function format_combined_binary_header_n 

pub fn format_combined_binary_header_n(
    path: &str,
    parent_oids: &[ObjectId],
    result_oid: &ObjectId,
    abbrev: usize,
    use_cc_word: bool,
) -> String

index line for N-parent combined/binary diffs (p1,p2,...pn..result).