ui-log: Colorize commit graph

Use the existing coloring logic in Git's graph code to color the lines
between commits in the commit graph.

Whereas Git normally uses ANSI color escapes to produce colors, we here
use graph_set_column_colors() to replace those with HTML color escapes,
that embed the graph lines in <span> tags that apply the desired color
using CSS.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>