Correctly collapse indented-<pre> whitespace.

<pre>'s whitespace collapsing behavior should be based on the first line
of the block, rather than on the indentation of the opening <pre> tag.
That is:

  Text text text.

    <pre>

    </pre>

Should generate:

  <p>Text text text.</p>
  <pre>
  Pre pre pre
  </pre>

This patch modifies `transformPre()` to grab the prefix from the first
inner line of the block, rather than |startLine|.