revlog: reduce memory usage in addgroup
- use a buffer to extract the delta from a chunk
- avoid concatenating to a compressed delta
- use a buffer to directly extra full text from a trivial delta
- delete chunk and delta objects after use
<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#</a></td>
</tr>
</table>