Convert plugin not handling tracks with the same filename

Hi,

I use the convert plugin to maintain a separate mp3 encoded copy of my library. I have [this album ](https://musicbrainz.org/release/318462e7-b00d-4612-a4f1-a3c320af0fe8) which has tracks with the same name on separate discs (see track 1 on discs 1 and 3 for example). The convert plugin doesn't convert the track on the third disc because it says the target file exists, but it doesn't. In the source directory the file for disc 3 ends with `.1.flac`.

Source directory:
```
andrew@tv:~/beets$ ls /mnt/nfs/music_hq/The\ Beatles/The\ Beatles\ and\ Esher\ Demos/01*
'/mnt/nfs/music_hq/The Beatles/The Beatles and Esher Demos/01 Back in the U.S.S.R_.1.flac' '/mnt/nfs/music_hq/The Beatles/The Beatles and Esher Demos/01 Birthday.flac'
'/mnt/nfs/music_hq/The Beatles/The Beatles and Esher Demos/01 Back in the U.S.S.R_.flac'
```

Target directory:
```
andrew@tv:~/beets$ ls /mnt/nfs/music/The\ Beatles/The\ Beatles\ and\ Esher\ Demos/01*
'/mnt/nfs/music/The Beatles/The Beatles and Esher Demos/01 Back in the U.S.S.R_.mp3' '/mnt/nfs/music/The Beatles/The Beatles and Esher Demos/01 Birthday.mp3'
```

Output from beets command (selected lines only):
```
andrew@tv:~/beets$ beet convert -y The Beatles The Beatles and Esher Demos
The Beatles - The Beatles and Esher Demos - Back in the U.S.S.R.
...
The Beatles - The Beatles and Esher Demos - Back in the U.S.S.R.
...
convert: Skipping /mnt/nfs/music_hq/The Beatles/The Beatles and Esher Demos/01 Back in the U.S.S.R_.flac (target file exists)
...
convert: Skipping /mnt/nfs/music_hq/The Beatles/The Beatles and Esher Demos/01 Back in the U.S.S.R_.1.flac (target file exists)
```

Convert plugin convert:
```
convert:
dest: /mnt/nfs/music/
max_bitrate: 200
formats:
mp3:
command: ffmpeg -i $source -y -vn -aq 2 -map_metadata 0 -id3v2_version 3 $dest
extension: mp3
```