expand --pretty=format color options

expand --pretty=format color options

Currently, the only colors available to --pretty=format
users are red, green, and blue. Rather than expand it with a
few new colors, this patch makes the usual config color
syntax available, including more colors, backgrounds, and
attributes.

Because colors are no longer bounded to a single word (e.g.,
%Cred), this uses a more advanced syntax that features a
beginning and end delimiter (but the old syntax still
works). So you can now do:

  git log --pretty=tformat:'%C(yellow)%h%C(reset) %s'

to emulate --pretty=oneline, or even

  git log --pretty=tformat:'%C(cyan magenta bold)%s%C(reset)'

if you want to relive the awesomeness of 4-color CGA.

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