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

While playing first track of cue file on satellite setup, the tag of
currentsong will loss. This is due to `chunk->tag` overwriting song tag
in `PlayerControl::PlayChunk`. In local cue files, `song_tag` is used to
form `stream_tag` and merged into `chunk->tag`, but `song_tag` only
works for local files. So the `chunk->tag` is decided by decoded tag
from audio files, which in cue's case is the whole album file and
doesn't contain track metadata.

This commit fixes the issue by merging song's tag with `chunk->tag`
(with `chunk->tag` prioritized) in `PlayerControl::LockUpdateSongTag`,
only when the song's `end_time` is not zero. As the `end_time` will
only be set on cue virtual songs or with `rangeid` command, i.e. songs
represented by portion of a whole file, where metadata of the portion
and whole file may differ. The impact to current behaviour is minimized:
changes only happen when users use `rangeid` command on a remote song,
and they expect that song tags in database should be completely
overwritten with tags provided by remote stream instead of merged.

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(song.GetTag(), new_tag));
+	else
+		song.SetTag(new_tag);
 
 	LockSetTaggedSong(song);