This disables moving the bonkered moving of the current song to
a (negative) offset of itself (introduced in the last commit).

This also short circuits no-op moves when (from == to) and
avoid needless increasing of the playlist version and causes
clients to issue pointless no-op plchanges commands.

git-svn-id: https://svn.musicpd.org/mpd/trunk@7153 09075e82-0dd4-0310-85a5-a0d7c8717e4f