dm6: bump version to 0.20101024, more comprehensive garbage collector

dm6: bump version to 0.20101024, more comprehensive garbage collector

The garbage collector now counts all links to files in the link directory,
and removes files where all links are accounted for within the link
directory.

This enables possible future enhancements, like having something
analogous to --timestamps and --access in faster-dupemerge, without
requiring a special case for each one.