default color.status.branch to "same as header"

default color.status.branch to "same as header"

This gives it the same behavior as we had prior to 1d28232
(status: show branchname with a configurable color).

To do this we need the concept of a "NIL" color, which is
provided by color.[ch]. The implementation is very simple;
in particular, there are no precautions taken against code
accidentally printing the NIL. This should be fine in
practice because:

  1. You can't input a NIL color in the config, so it must
     come from the in-code defaults. Which means it is up
     the client code to handle the NILs it defines.

  2. If we do ever print a NIL, it will be obvious what the
     problem is, and the bug can be fixed.

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