Well, at least in Git, the commit message already has two sections.

AFAIK, the first line is supposed to be the a very brief summary, while the other lines may contain additional information and can sometimes make up a pretty long text. Lots of tools make use of this convention and only show the first line if no detail information is needed.

Most tools that show the history only show the first line of each commit.

But I agree with you, this still assumes that long commit messages are rare and not that almost every commit has a huge message. Also, "long" doesn't mean you should put a novel in there.