Merge branch 'jk/colors'

"diff-highlight" filter (in contrib/) allows its color output
to be customized via configuration variables.

* jk/colors:
  parse_color: drop COLOR_BACKGROUND macro
  diff-highlight: allow configurable colors
  parse_color: recognize "no$foo" to clear the $foo attribute
  parse_color: support 24-bit RGB values
  parse_color: refactor color storage

diff --cc Documentation/config.txt
Simple merge

diff --cc contrib/diff-highlight/diff-highlight
Simple merge