commit: write last-message.txt with no content modifications.
This assumes the user will use either a sane editor (anything but
Notepad) or the same editor again: even Notepad should be able to read
what it previously wrote.
Mercurial's documentation is kept in reStructuredText format, which is
a simple plain text format that's easy to read and edit:
http://docutils.sourceforge.net/rst.html
It's also convertible to a variety of other formats including standard
UNIX man page format and HTML. You'll need to install Docutils:
http://docutils.sourceforge.net/
Use the Makefile in this directory to generate the man and HTML pages.