Also fix double loading of files clicked in ToC before they were loaded.

In future, we can detect which file is looked at by user and load that with priority too.

Array is not the best data structure for priority queue but it's not a big deal as we'd usually reorder it just once.

Added sleep(5) to DifferentialChangesetViewController, decreased load_limit to 1, clicked on third file in ToC, saw it loaded before the second.