Function vimdiff_merge_argv_with_base 

pub fn vimdiff_merge_argv_with_base(
    final_cmd: &str,
    local: &str,
    base: &str,
    remote: &str,
    merged: &str,
) -> Vec<String>

Builds argv for vim -f -c '...' LOCAL BASE REMOTE MERGED (base present), matching Git’s merge_cmd eval.