convert: disable current --filemap support
The --filemap support in hg convert doesn't handle merges correctly.
(And after 33015dac5df5 I managed to break it even for simple cases
where we don't want the first revision.)
#header#
<title>Mercurial Error</title>
</head>
<body>
<h2>Mercurial Error</h2>
<p>
An error occured while processing your request:
</p>
<p>
#error|escape#
</p>
#footer#