add prechangegroup and pretxnchangegroup hooks.
prechangegroup lets you stop push, pull or unbundle before it begins.
pretxnchangegroup lets you inspect changegroup before transaction is
committed, and roll back if you not like it.
#header#
<title>Mercurial repositories index</title>
</head>
<body>
<h2>Mercurial Repositories</h2>
<table>
<tr>
<td>Name</td>
<td>Description</td>
<td>Contact</td>
<td>Last change</td>
<td> </td>
<tr>
#entries%indexentry#
</table>
#footer#