git_config_colorbool: refactor stdout_is_tty handling

git_config_colorbool: refactor stdout_is_tty handling

Usually this function figures out for itself whether stdout
is a tty. However, it has an extra parameter just to allow
git-config to override the auto-detection for its
--get-colorbool option.

Instead of an extra parameter, let's just use a global
variable. This makes calling easier in the common case, and
will make refactoring the colorbool code much simpler.

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