Fixing diff-comments and close by empty diff

Fixing bug for diff-comments by html formatter:
it was:
```html
<li class="del"><del><strong>--</strong> /tmp/diffy20130605-4371-<strong>kywx0v</strong>-0  2013-06-05 13:58:53.000000000 +0200</del></li>
<li class="ins"><ins><strong>++</strong> /tmp/diffy20130605-4371-<strong>1arvip</strong>-0  2013-06-05 13:58:53.000000000 +0200</ins></li>
```
adding row to close <div class="diff"/> directly if the diff is empty -> allow-empty-diff. Better xhtml and better for parsing.