convert: fix svn crash when svn.ra.get_log calls back with orig_paths=None get_log started calling back with orig_paths=None on Fedora 20 with subversion-1.8.3. That broke test-convert-svn-source.t . There used to be some handling of that situation until e8f4e40f285a apparently broke it. This patch restores what seems to be the most obvious handling of the situation.