bash_completion: use hg --debug help to get the list of debug commands.
Also, try completing with the debug commands only when there's no other
candidates.
Based on an idea by TK Soh.
#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#