changelogv2: introduce a "changelogv2" feature
Right now, this means using revlogv2, but only for the changelog. We will have
the format more unique in future changesets.
Differential Revision: https://phab.mercurial-scm.org/D10660
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.