Shade and zebra styles for annotate.

This patch adds two more annotate styles: zebra and shade.

Zebra alternates the line background colour according to the patch, just like
most CVS viewers do.

Shade is a lot more fun: it gives each patch a shade of a colour, the newer
the brighter, so newer patches (which are often the ones you're intrested in)
are easier to spot. It could use a little tuning on some constants to make it
look better, but it's pretty usable as is, and it can be done later if needed.
Thanks to Max Battcher for the idea.