automatically generate initial in-reply-to?

All,

My personal preference is an overview message of "[PATCH 0/n] xxxx"
to be a thread topic (first message in this thread) and then each of
[PATCH m/n] with In-Reply-To: pointing to this first message instead
of pointing to the previous one. So I came up with the following
command:

git-send-email --compose --no-chain-reply-to patch/to/send/*

I then was prompted with:

Message-ID to be used as In-Reply-To for the first email?

I tried to leave this empty and also input some random strings,
both failed me by setting all In-Reply-To: to the same thing.

So here're two questions:

1. what's the correct command to do so?

2. input in-reply-to manually does not sound like a good idea,
is there some way I can come up with an automatically
created one?


-- 
Cheers
- eric