stablerange: fix merge slicing when range has multiple roots
The first element in the bottom set is not necessarly the one with the lowest
revision. We now properly compute and use the minimum value.
This software was downloaded from
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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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