Set Message-ID when sending package request emails

When sending the first notification email that belongs to a certain
package request, set the Message-ID instead of the In-Reply-To and
References headers. This is not only more RFC compliant but also fixes
thread views in several clients, such as Pipermail.
Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>

web/lib/pkgreqfuncs.inc.php

@@ -109,8 +109,7 @@ function pkgreq_file($ids, $type, $merge_into, $comments) {
 	}
 	$thread_id = "<pkg-request-" . $request_id . "@aur.archlinux.org>";
 	$headers .= "From: notify@aur.archlinux.org\r\n" .
-		    "In-Reply-To: $thread_id\r\n" .
-		    "References: $thread_id\r\n" .
+		    "Message-ID: $thread_id\r\n" .
 		    "X-Mailer: AUR";
 	@mail($AUR_REQUEST_ML, "[PRQ#" . $request_id . "] " . ucfirst($type) .
 			       " Request for " .  $row['Name'], $body,