compat: make obshistory._getdifflines compatible with mercurial 4.1
On Mercurial version 4.1 and lower, patch.diff seems to returns an empty list
when there is no change. Mercurial 4.2 returns None so we add a if for this
case.
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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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2 can be found in `/usr/share/common-licenses/GPL-2'.