In preparation for adding `--type=color` to the `git-config(1)` builtin,
let's introduce a color parsing utility, `git_config_color` in a similar
fashion to `git_config_<type>`.
Signed-off-by: NTaylor Blau <me@ttaylorr.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>