Bug#212920: recovery from failed runs

[Petter Reinholdtsen <pere@hungry.com>]

[Joey Hess]
> Before debootstrap is run, list all files in /target.
> If the run fails, take the list agian, and compare the lists to find all
> files added during the run. Remove these.

I believe we should generate this list every time debootstrap is
executed.  Then we can check if this list exists when running
debootstrap, and remove the files listed there before we run
debootstrap again.  This would make it possible to select
base-installer several times.