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>
diff --git a/builtin/commit.c b/builtin/commit.c
index e1af9b1..295803a 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1144,7 +1144,7 @@
 		return 0;
 	}
 	if (!strcmp(k, "status.color") || !strcmp(k, "color.status")) {
-		s->use_color = git_config_colorbool(k, v, -1);
+		s->use_color = git_config_colorbool(k, v);
 		return 0;
 	}
 	if (!prefixcmp(k, "status.color.") || !prefixcmp(k, "color.status.")) {