The logic for the diff complexity icon in the diff viewer is now its own

view, making it easier to reuse. It's also a bit smarter now, in that it

doesn't require a number of replaces or total lines to work (though

providing this information adds more detail to the icon).
With this comes a performance improvement. Previously, each icon was

calculating the necessary colors for itself, but now they're calculated

only the first time an icon is used, and cached for future icons.

Loaded the diff viewer and saw the icons.
Used the icon in another change, without all the information available

during diff rendering, and still saw a decent icon.