color: add support for 12-bit RGB colors

color: add support for 12-bit RGB colors

RGB color parsing currently supports 24-bit values in the form #RRGGBB.

As in Cascading Style Sheets (CSS [1]), also allow to specify an RGB color
using only three digits with #RGB.

In this shortened form, each of the digits is – again, as in CSS –
duplicated to convert the color to 24 bits, e.g. #f1b specifies the same
color as #ff11bb.

In color.h, remove the '0x' prefix in the example to match the actual
syntax.

[1] https://developer.mozilla.org/en-US/docs/Web/CSS/hex-color

Signed-off-by: Beat Bolli <dev+git@drbeat.li>
Signed-off-by: Junio C Hamano <gitster@pobox.com>