command: play the correct entry with loadfile ... append-play

The playlist may be non-empty even if the player is idle. Instead of playing the
first entry, play the entry that was just added.

(again, I don't know if the current behavior was on purpose, but I think it makes more sense this way, and it would make some hairy parts in grooved, and probably other projects using libmpv, much simpler. I also think it wouldn't conflict with the original use case in #950, since the guy explicitly mentioned an empty playlist).