Update convert help text
Add Mercurial as a source format, clarify that the include directive triggers the exclusion of all not explicitely included files/dirs and use MAPFILE instead of revmapfile in the text, following the short message convention.
#motd#
<div class="logo">
powered by<br/>
<a href="http://www.selenic.com/mercurial/">mercurial</a>
</div>
</body>
</html>