Fix bug with `display-xexpr`.

Although it was properly not indenting <pre> elements, it was failing
to account for the fact that _other_ elements could be nested _inside_
<pre> elements. Although our raw Markdown xexprs didn't create any
such nested elements, and the bug didn't show up that way, it did show
up when e.g. Frog added such elements created by Pygments (e.g. table
and span elements).