diff-highlight: allow configurable colors

diff-highlight: allow configurable colors

Until now, the highlighting colors were hard-coded in the
script (as "reverse" and "noreverse"), and you had to edit
the script to change them. This patch teaches diff-highlight
to read from color.diff-highlight.* to set them.

In addition, it expands the possiblities considerably by
adding two features:

  1. Old/new lines can be colored independently (so you can
     use a color scheme that complements existing line
     coloring).

  2. Normal, unhighlighted parts of the lines can be colored,
     too. Technically this can be done by separately
     configuring color.diff.old/new and matching it to your
     diff-highlight colors. But you may want a different
     look for your highlighted diffs versus your regular
     diffs.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>