send-email: respect in-reply-to regardless of threading

git-send-email supports the --in-reply-to option even with
--no-thread.  However, the code that adds the relevant mail headers
was guarded by a test for --thread.

Remove the test, so that the user's choice is respected.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -727,7 +727,7 @@ sub send_message
 Message-Id: $message_id
 X-Mailer: git-send-email $gitversion
 ";
-	if ($thread && $reply_to) {
+	if ($reply_to) {
 
 		$header .= "In-Reply-To: $reply_to\n";
 		$header .= "References: $references\n";