convert: auto (#212) now transcodes to /tmp
convert: auto (#212) now transcodes to /tmp

This avoids naming conflicts in the source directory. In particular, when
encoding MP3 -> MP3, the previous scheme would overwrite the original file
(and hang ffmpeg waiting for input). This should also work in
situations where the source directory is read-only.

google-code-export/beets,google-code-export/beets,google-code-export/beets