Fix whitespace_pre with incremental reflow turned on.

This implements fragment merging, in order to incrementally reflow linebroken
text. This makes the `whitespace_pre.html` reftest pass with incremental reflow
turned on with `-i`.
It'd be slightly uglier because of having to `left_info.range.extend_by(right_info.range.length() + CharIndex(1));` in the `true` case.