hgweb: use "nonempty" filter in templates for the changelog message
At least subversion allows empty commit logs which, after conversion,
show up as empty fields in hgweb and don't allow selecting the
changeset. Using "nonempty" as filter, "(none)" is used to get
clickable links for empty commit messages.
<tr class="parity{parity}">
<td class="age">{date|age}</td>
<td class="author">{author|person}</td>
<td class="description"><a href="{url}rev/{node|short}{sessionvars%urlparameter}">{desc|strip|firstline|escape|nonempty}</a>{inbranch%changelogbranchname}{branches%changelogbranchhead}{tags%changelogtag}</td>
</tr>