New Message Extension

[Adrian Buehlmann]

I've created a new extension:

http://mercurial.selenic.com/wiki/MessageExtension

It allows to preset the commit message (stored in .hg/message), which
is then preloaded into the editor on 'hg ci'.

Note that 'hg ci' does not change .hg/message, so rollback and
recommit is no problem.

The idea is that the user can note in advance what he is doing.
Might also be helpful when working with lots of clones.