We want to delete multiple backups in one go to optimize tarsnap bandwidth cost
See: http://mail.tarsnap.com/tarsnap-users/msg00590.html
optimize prune_parts deletes
Merge pull request #2 from pjv/optimize-deletes

Use multiple <-f archive> arguments on a single invocation of tarsnap to save money (rather than deleting each archive with a separate invocation), assuming tarsnp version is > 1.0.27 which was released in June 2010.