parse_color: refactor color storage

parse_color: refactor color storage

When we parse a color name like "red" into its ANSI color
value, we pack the storage into a single int that may take
on many values:

  1. If it's "-2", no value has been specified.

  2. If it's "-1", the value is "normal" (i.e., no color).

  3. If it's 0 through 7, the value is a standard ANSI
     color.

  4. If it's larger (up to 255), it is a 256-color extended
     value.

Given these magic numbers, it is often hard to see what is
going on in the code. Let's refactor this into a struct with
a flag that tells which scheme we are using, along with a
numeric value. This is more verbose, but should hopefully be
simpler to follow. It will also allow us to easily add
support for more schemes, like 24-bit RGB values.

The result is also slightly less efficient to store, but
that's OK; we only store this intermediate state during the
parse, after which we write out the actual ANSI bytes.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>