notify: Fix references in request notifications

When sending notifications upon request creation, set an initial message
ID instead of setting the Reply-To and References headers.

This used to work but the behavior was unintentionally changed in
9746a654

 (Port notification routines to Python, 2015-06-27).

Fixes FS#46645.
Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>

scripts/notify.py

@@ -29,6 +29,9 @@ reply_to = config.get('notifications', 'reply-to')
 def headers_cc(cclist):
     return {'Cc': str.join(', ', cclist)}
 
+def headers_msgid(thread_id):
+    return {'Message-ID': thread_id}
+
 def headers_reply(thread_id):
     return {'In-Reply-To': thread_id, 'References': thread_id}
 
@@ -210,7 +213,7 @@ def request_open(cur, uid, reqid, reqtype, pkgbase_id, merge_into=None):
         refs = '[1] ' + user_uri + '\n'
         refs += '[2] ' + pkgbase_uri + '\n'
     thread_id = '<pkg-request-' + reqid + '@aur.archlinux.org>'
-    headers = headers_reply(thread_id) + headers_cc(cc)
+    headers = headers_msgid(thread_id) + headers_cc(cc)
 
     send_notification(to, subject, body, refs, headers)