built-in add -i: color the header in the `status` command
For simplicity, we only implemented the `status` command without colors.
This patch starts adding color, matching what the Perl script
`git-add--interactive.perl` does.
Original-Patch-By: Daniel Ferreira <bnmvco@gmail.com> Signed-off-by: Slavica Đukić <slawica92@hotmail.com> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>