convert_git: avoid returning two entries for the same file in getchanges
This could happen in merge changesets if the merged file was different
from both parents.
#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#