Function process_trailers 

pub fn process_trailers(
    input: &str,
    opts: &ProcessTrailerOptions,
    new_trailer_args: &[NewTrailerArg],
    git_dir: Option<&Path>,
) -> String

Process a commit message: parse trailer block, apply config and --trailer args, emit result.

git_dir selects which repository config to load (with standard cascade). When None, only non-repo config layers are used (matches git interpret-trailers outside a repo).