For querying tags, the real song URI should be used, because if the
(display) URI is different, requesting it will not produce a usable
response. This is a theoretical problem because none of the existing
playlist plugins sets the real_uri.
This requires changing the URI comparison in playlist::TagModified().
Closes https://github.com/MusicPlayerDaemon/MPD/issues/1154
c3f9b38c97