compat: use mergetoolopts from commands.py
Mercurial <= 4.2 do not have cmdutils.py yet.
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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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