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>

git-send-email.perl

@@ -1137,7 +1137,8 @@ foreach my $t (@files) {
 	send_message();
 
 	# set up for the next message
-	if ($chain_reply_to || !defined $reply_to || length($reply_to) == 0) {
+	if ($thread &&
+		($chain_reply_to || !defined $reply_to || length($reply_to) == 0)) {
 		$reply_to = $message_id;
 		if (length $references > 0) {
 			$references .= "\n $message_id";

t/t9001-send-email.sh

@@ -621,7 +621,7 @@ test_expect_success 'in-reply-to but no threading' '
 	grep "In-Reply-To: <in-reply-id@example.com>"
 '
 
-test_expect_failure 'no in-reply-to and no threading' '
+test_expect_success 'no in-reply-to and no threading' '
 	git send-email \
 		--dry-run \
 		--from="Example <nobody@example.com>" \