Implement `bak list --relpaths`

[ChanceNCounter]

The relevant config value is present, but the functionality is not. With this config value or CLI flag, bak list should display original paths relative to $PWD, rather than their absolute paths. This is ugly when you do it with your full bak list, but it can make the table easier to look at when you're doing it on a specific file, while working on that file.