stablesort: move parent range computation into its own method
Now that we have running code, we'll need to avoid too deep recursion. We
extract the piece dealing with the parent range before we write more code to
warm it.
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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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