Function message_ends_with_trailer 

pub fn message_ends_with_trailer(msg: &str, config: &ConfigSet) -> bool

Whether the final non-blank line of msg looks like a trailer (per trailer.c rules from config). Used to decide whether an appended Signed-off-by: joins the existing trailer block (single \n) or starts a new paragraph (\n\n), matching Git’s append_signoff.