send-email: fix threaded mails without chain-reply-to

An earlier commit 15da108 ("send-email: 'References:' should only
reference what is sent", 2009-04-13) broke logic to set up threading
information for the next message by rewriting "!" to "not" without
understanding the precedence rules of the language.

Namely,

    ! defined $reply_to || length($reply_to) == 0

was changed to

    not defined $reply_to || length($reply_to) == 0

which is

    not (defined $reply_to || length($reply_to) == 0)

and different from what was intended, which is

    (not defined $reply_to) || (length($reply_to) == 0)

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

git-send-email.perl

@@ -1150,7 +1150,8 @@ sub send_message
 	my $message_was_sent = send_message();
 
 	# set up for the next message
-	if ($message_was_sent and $chain_reply_to || not defined $reply_to || length($reply_to) == 0) {
+	if ($message_was_sent &&
+		($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 'threading but no chain-reply-to' '
+test_expect_success 'threading but no chain-reply-to' '
 	git send-email \
 		--dry-run \
 		--from="Example <nobody@example.com>" \