Do not auto-create the bup repository

Fixes https://github.com/anarcat/bup-cron/issues/11
Merge pull request #12 from blueyed/do-not-autocreate-bup-dir

do not auto-create the bup repository

the idea here is to avoid creating a whole directory hierarchy, especially because the bup repo maybe behind a mountpoint that may have failed to mount in a wrapper script. in that case we don't want to start creating a bunch of directories in `/`.