script for determining space consumed by increments

[dean gaudet]

p.s. on linux, this script, and rdiff-backup will run faster if you mount
your backup partition with the "noatime" mount option.  i know other
unixes have similar options, but i don't know the syntax.

noatime disables the update of atime every time a file is accessed -- it
causes vast amount of disk write traffic to update metadata when you are
iterating over an entire filesystem.