> Enhance git-send-email to recognize executable scripts passed to --to,
> --cc, or --bcc. When a recipient argument is an executable file, run it
> in a subshell and use its output as the recipient list.
>
> This allows users to automate recipient selection using scripts like
> get_maintainer.pl in the Linux kernel. The script is called with the
> corresponding flag (--to, --cc, or --bcc) and all remaining command-line
> arguments (typically the patches being sent).
>
> Modify execute_cmd() to support multiple arguments safely using
> quotemeta. Add test cases to verify the new functionality and ensure
> arguments are correctly passed to the scripts.
>
> Co-developed-by: Gemini CLI <gemini-cli@google.com>