evolve: extract cleanup logic in the evolve function
We are going to need to reuse the cleanup logic when introducing --rev, so
we extract it in a method to avoid code duplication.
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Copyright 2011 Peter Arrenbrecht <peter.arrenbrecht@gmail.com>
Logilab SA <contact@logilab.fr>
Pierre-Yves David <pierre-yves.david@ens-lyon.org>
Patrick Mezard <patrick@mezard.eu>
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