refine NNTP Subject header use

Don't prepend it to the message if this is a followup post.
There's no need to put "Re: previous message subject" into
every followup post.

iris/backend.go

@@ -249,12 +249,14 @@ func msgToIris(article *nntp.Article) (*irisMsg, error) {
 	var msg irisMsg
 	msg.Data.Author = irisAuthor(article.Header.Get("From"))
 	msg.Data.Timestamp = postTime
-	msg.Data.Message = irisBody(article.Header.Get("Subject"), string(body))
 	refs := article.Header.Get("References")
 	if refs != "" {
 		spl := strings.SplitN(refs, " ", 2)
 		ref := fromMsgID(spl[0])
 		msg.Data.Parent = &ref
+		msg.Data.Message = string(body)
+	} else {
+		msg.Data.Message = irisBody(article.Header.Get("Subject"), string(body))
 	}
 
 	hash, err := msg.calcHash()