cmdutil: add special string that ignores rest of text
Similar to git, we add a special string:
HG: ------------------------ >8 ------------------------
that means anything below it is ignored in a commit message.
This is helpful for integrating with third-party tools that display the
mercurial (__VERSION__) __CODENAME__; urgency=medium
* Automated build performed by upstream.
-- Mercurial Devel <mercurial-devel@selenic.com> __DATE__