git-gui: Improve the coloring in blame viewer

git-gui: Improve the coloring in blame viewer

The git-gui blame viewer has always been ugly as s**t.  Linus Torvalds
suggested the coloring scheme I'm using here, which is two different
shades of grey for the background colors, and black text on a pale green
background for the currently selected/focused commit.

The difference is a massive improvement.  The interface no longer will
cause seizures in people who are prone to that sort of thing.  It no
longer uses a very offensive hot pink.  The green being current actually
makes sense.  And not having the background of the other non-current
lines change when you change the current commit is really a big deal.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>