diff-highlight: fetch all config with one process

diff-highlight: fetch all config with one process

When diff-highlight was written, there was no way to fetch multiple
config keys _and_ have them interpreted as colors. So we were stuck
with either invoking git-config once for each config key, or fetching
them all and converting human-readable color names into ANSI codes
ourselves.

I chose the former, but it means that diff-highlight kicks off 6
git-config processes (even if you haven't configured anything, it has to
check each one).

But since Git 2.18.0, we can do:

   git config --type=color --get-regexp=^color\.diff-highlight\.

to get all of them in one shot.

Note that any callers which pass in colors directly to the module via
@OLD_HIGHLIGHT and @NEW_HIGHLIGHT (like diff-so-fancy plans to do) are
unaffected; those colors suppress any config lookup we'd do ourselves.

You can see the effect like:

  # diff-highlight suppresses git-config's stderr, so dump
  # trace through descriptor 3
  git show d1f33c753d | GIT_TRACE=3 diff-highlight 3>&2 >/dev/null

which drops from 6 lines down to 1.

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