Add and use generic name->id mapping code for color slot parsing

Add and use generic name->id mapping code for color slot parsing

Instead of hard coding the name-to-id mapping in C code, keep it in an
array and use a common function to do the parsing. This reduces code
and also allows us to list all possible color slots later.

This starts using C99 designated initializers more for convenience
(the first designated initializers have been introduced in builtin/clean.c
for some time without complaints)

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>