fix incorrect warning when walking on a particular rev
when a directory was given instead of a file it reported
incorrectly 'No such file or directory in rev <rev>'
we test if the file is a prefix directory
#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#