(gnus-summary-refer-thread): Don't clobber unread articles

This fixes a bug where `A T' causes "random" articles to become marked
as read.

* lisp/gnus/gnus-sum.el (gnus-summary-refer-thread): Make sure
gnus-newsgroup-unreads remains sorted.

lisp/gnus/gnus-sum.el

@@ -9068,22 +9068,24 @@ non-numeric or nil fetch the number specified by the
 		       (regexp-opt ',(append refs (list id subject)))))))
 	      (gnus-fetch-headers (list last) (if (numberp limit)
 						  (* 2 limit) limit) t))))
-	 article-ids)
+	 article-ids new-unreads)
     (when (listp new-headers)
       (dolist (header new-headers)
-	(push (mail-header-number header) article-ids)
-	(when (member (mail-header-number header) gnus-newsgroup-unselected)
-          (push (mail-header-number header) gnus-newsgroup-unreads)
-          (setq gnus-newsgroup-unselected
-                (delete (mail-header-number header)
-			gnus-newsgroup-unselected))))
+	(push (mail-header-number header) article-ids))
+      (setq article-ids (nreverse article-ids))
+      (setq new-unreads
+	    (gnus-sorted-intersection gnus-newsgroup-unselected article-ids))
+      (setq gnus-newsgroup-unselected
+	    (gnus-sorted-ndifference gnus-newsgroup-unselected new-unreads))
+      (setq gnus-newsgroup-unreads
+	    (gnus-sorted-nunion gnus-newsgroup-unreads new-unreads))
       (setq gnus-newsgroup-headers
             (gnus-delete-duplicate-headers
              (gnus-merge
               'list gnus-newsgroup-headers new-headers
               'gnus-article-sort-by-number)))
       (setq gnus-newsgroup-articles
-      	    (gnus-sorted-nunion gnus-newsgroup-articles (nreverse article-ids)))
+	    (gnus-sorted-nunion gnus-newsgroup-articles article-ids))
       (gnus-summary-limit-include-thread id)))
   (gnus-summary-show-thread))