Function format_parent_patch 

pub fn format_parent_patch(
    git_dir: &Path,
    config: &ConfigSet,
    odb: &Odb,
    path: &str,
    parent_tree: &ObjectId,
    result_tree: &ObjectId,
    abbrev: usize,
    context: usize,
    use_textconv: bool,
) -> Option<String>

diff --git against parent p for merge commit -m output.