walkchangerevs: move 'add' to callback
Now walkchangerevs is a simple iterator over contexts
<table class="slogEntry parity{parity}">
<tr>
<td class="age">{date|age}</td>
<td class="author">{author|person}</td>
<td class="node"><a href="{url}rev/{node|short}{sessionvars%urlparameter}">{desc|strip|firstline|escape|nonempty}</a></td>
</tr>
</table>