git-gui: Improve annotated file display.

git-gui: Improve annotated file display.

Rather than trying to mark the background color of the line numbers
to show which lines have annotated data loaded, we now show a ruler
between the line numbers and the file data.  This ruler is just 1
character wide and its background color is set to grey to denote
which lines have annotation ready.  I had to make this change as I
kept loosing the annotation marker when a line was no longer colored
as part of the current selection.

We now color the lines blamed on the current commit in yellow, the
lines in the commit which came after (descendant) in red (hotter,
less tested) and the lines in the commit before (ancestor) in blue
(cooler, better tested).

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