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:

```html
  Text text text.

    <pre>
        Pre pre pre
    </pre>
```

Should generate:

```html
  <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|.