Merge branch 'bb/rgb-12-bit-colors'

The color parsing code learned to handle 12-bit RGB colors, spelled
as "#RGB" (in addition to "#RRGGBB" that is already supported).

* bb/rgb-12-bit-colors:
  color: add support for 12-bit RGB colors
  t/t4026-color: add test coverage for invalid RGB colors
  t/t4026-color: remove an extra double quote character