3e0c4ff (send-email: respect in-reply-to regardless of threading,
2009-03-01) fixed the handling of the In-Reply-To header when both
--no-thread and --in-reply-to are in effect. Add a test for it.
Signed-off-by: Thomas Rast <trast@student.ethz.ch> Signed-off-by: Junio C Hamano <gitster@pobox.com>