send-email: fix non-threaded mails

send-email: fix non-threaded mails

After commit 3e0c4ff (send-email: respect in-reply-to regardless of
threading, 2009-03-01) the variable $thread was only used for prompting
for an "In-Reply-To", but not for controlling whether the "In-Reply-To"
and "References" fields should be written into the email.

Thus these fields were always used beginning with the second mail and it
was not possible to produce non-threaded mails anymore.

However, a later commit 15da108 ("send-email: 'References:' should only
reference what is sent", 2009-04-13) introduced a regression with the
side effect to make non-threaded mails possible again, but only when
--no-chain-reply-to was used.

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>