pick: reduce configoverride() scope
Merge tool is only needed for merge.graft() (it's what graft in core hg does),
so let's make the scope of the ui.configoverride() narrower.
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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