Pierre-Yves David <pierre-yves.david@ens-lyon.org> [Wed, 15 Mar 2017 15:11:52 -0700] rev 31445
filemerge: explicitly tests for None
Changeset 758526333dec removed the mutable default value, but did not explicitly
tested for None. Such implicit testing can introduce semantic and performance
issue. We move to an explicit testing for None as recommended by PEP8:
https://www.python.org/dev/peps/pep-0008/#programming-recommendations