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.
#header#
<title>Mercurial repositories index</title>
</head>
<body>
<h2>Mercurial Repositories</h2>
The specified repository "#repo|escape#" is unknown, sorry.
Please go back to the main repository list page.
#footer#