git-gui: Use three colors for the blame viewer background

git-gui: Use three colors for the blame viewer background

To prevent neighboring lines that are different commits from using
the same background color we now use 3 colors and assign them
by selecting the color that is not used before or after the line
in question.  We still color "on the fly" as we receive hunks from
git-blame, but we delay our color decisions until we are getting
the original location data (the slower -M -C -C pass) as that is
usually more fine-grained than the current location data.

Credit goes to Martin Waitz for the tri-coloring concept.

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