git-gui: Assign background colors to each blame hunk.
To help the user visually see which lines are associated with each other
in the file we attempt to sign a unique background color to each commit
and then render all text associated with that commit using that color.
This works out OK for a file which has very few commits in it; but
most files don't have that property.
What we really need to do is look at what colors are used by our
neighboring commits (if known yet) and pick a color which does not
conflict with our neighbor. If we have run out of colors then we
should force our neighbor to recolor too. Yes, its the graph coloring
problem.