doc fix: hg tags defaults to current parent revision, too.
Additionally use better language for the help text for tag,
cat and manifest.
#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#