player/Thread: merge tag for songs have range instead of replacing with chunk->tag

Author: datasone

src/player/Thread.cxx

@@ -903,7 +903,10 @@ PlayerControl::LockUpdateSongTag(DetachedSong &song,
 		   streams may change tags dynamically */
 		return;
 
-	song.SetTag(new_tag);
+	if (!song.GetEndTime().IsZero())
+		song.SetTag(Tag::Merge(new_tag, song.GetTag()));
+	else
+		song.SetTag(new_tag);
 
 	LockSetTaggedSong(song);