Reviewed by Adam Barth.

        side-by-side diffs in the code review tool
        https://bugs.webkit.org/show_bug.cgi?id=52019

        Support for conversion from the formatted diff to a side-by-side diff.
        Maintains comments and new comments can be added.

        The main architectural change is that Line elements are no longer necessarily
        siblings. Each physical line is now in a LineContainer and LineContainers are
        siblings. Each Line corresponds to a Line in the unified diff and has an id (e.g. line12).
        A Line can be a LineContainer or a child of a LineContainer.

        In this way, converting to side-by-side and, in the future, back to unified is non-lossy.

        * PrettyPatch/PrettyPatch.rb:
        * code-review.js:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@75295 268f45cc-cd09-0410-ab3c-d52691b4dbfc