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
# This config file points Mercurial at the system-wide certificate
# store from the ca-certificates package.
[web]
cacerts = /etc/ssl/certs/ca-certificates.crt