#7 pick_winnow_files can re-use vergroups

[psthomas]

FingerprintUtils.pick_winnow_files fails to populate selected_version_groups, but still uses it to (attempt to) reject duplicate vergroups. The result is suboptimal or entirely useless winnowing.

Also, pick_winnow_files contains a debug statement.

Thanks to: Tomas Ciml (Avast)