context: add missing b prefix
This fixes
mercurial/context.py:593: SyntaxError: cannot mix bytes and nonbytes literals (context.py, line 593)
in Python 3.
{header}
<title>Mercurial repository not found</title>
</head>
<body>
<h2>Mercurial repository not found</h2>
The specified repository "{repo|escape}" is unknown, sorry.
Please go back to the <a href="{url|urlescape}">main repository list page</a>.
{footer}