Add sub-message to a commit

[GOUJON Alexandre]

Hi,

I would like to add a 'sub'-message to a commit like this one
http://source.winehq.org/git/wine.git/commitdiff/dbd878639424071e1d4c6cfc9bcc75906a4b2b8a

I know that I can specify this text at the beginning of a patch I want 
to send to wine-patches (befor the '---').
But here, I'm interested in how to do that on the git server side.
git commit -a or git commit -m let me specify a 'global' commit message.
I googled some key words and RTFMs but didn't find anything relevant.

Any hints on how to do that ?
I can ask AJ directly but
1) it may be considered as rude and selfish
2) some people may be interested by the answer

Thanks in advance