qdiff: Maintain scroll position when refreshing

If you are reviewing a diff and scrolled 1/2 way down the diff window when hitting "refresh", we should attempt to scroll back to the same position as we were before, as that is likely to be where the changes we care about were made. This will be tricky to get right - the closest might be to remember the filename + the offset in that file and attempt to scroll back to that - even scrolling back to the top of the file being currently viewed would be an improvement.