keep-last.py starts with a python interpreter and does nothing
--help shows a help message
--directory parameter is mandatory
An error is raised if the directory specified by --directory does not exist
it just prints a (flat, non-recursive) list of all the found files
Keeper class encapsulates the logic to list the files in a directory
It extracts the list of the non-rev files
Each file is grouped with its revs
Each file is grouped with its revs ordered from the oldest to the newest
Only oldest revs are associated to each file, so that they could be deleted
It prints out the list of the oldest rev files that can be purged
Nothing to purge for +[file] filename