Colorize 'commit' lines in log ui

Colorize 'commit' lines in log ui

When paging through the output of git-whatchanged, the color cues help to
visually navigate within a diff. However, it is difficult to notice when a
new commit starts, because the commit and log are shown in the "normal"
color. This patch colorizes the 'commit' line, customizable through
diff.colors.commit and defaulting to yellow.

As a side effect, some of the diff color engine (slot enum, get_color) has
become accessible outside of diff.c.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>