Add a cleaner and more informative file index to the diff viewer.

The old file index was a bit of an ugly mess that never really got
cleaned up. There was no alignment, very little consistency, and it
didn't show much in the way of useful information.

This new one has a clean layout with some useful data.

When loading, a spinner appears to the left of the filename, instead of
after it.

The spinner is then replaced with a little pie chart icon representing
the complexity of the change. This is based on the line counts to the
file. The pie chart shows the inserts vs. deletes vs. replaces. The
center of the pie chart is white, representing the unchanged lines.
The larger the white center, the less of the file has changed. This
makes it really easy to see at a glance how big a change is to a file.

Then there's the filename, which links to the proper part of the diff.
Depending on the file, there may be additional information below the
file (renamed file info).

Then there's a list of chunks, in the order of the file. This is a bit
different from the pie graph, in that it acts as a way of seeing how
many chunks of changes (rather than relative numbers of lines) exist,
and jumping to them. Combined with the pie chart, it's easier to get a
good sense of how much effort a diff is going to be to review.

(We discussed possibly removing this in favor of just the graph, but I
think that it complements the pie chart well when trying to get a sense
for how complex a change will be to review, so I'd like to try it out in
real-world usage for a while.)

If the file is a binary file or deleted file, then a little status text
saying "Deleted" or "Binary file" will be displayed instead of the
chunks.

When failing to load a diff, the icon is replaced with a warning icon.

Tested with a variety of types of changes (as the screenshot demonstrates).
This covers:

* New files
* Deleted files
* Changed files (with different combinations of inserts, deletes and replaces)
* Moved/renamed files
* Binary files
* Diff load errors