Use file names for hash keys instead of the song ID.

[sanjoy]

MPD resets the ID counter when restarted, causing mingus to display bogus tracks after a restart. This patch works around the issue by hashing on the songs' file names.

[cmacrae]

Not sure what the implications of these changes would be for those (like me) using a streaming back-end for MPD, like Mopidy, would be. I don't know if Mopidy actually ends up writing files that represent the data being streamed, or not.

Anyway, just something to keep in mind.

Though "sanjoy commented on Sep 22, 2011"... I'm sure there's no danger of it being merged any time soon!