symmetric relations: replace bogus rql2sql translation by a hook The hook ensures X r Y => Y r X iff r is symmetric. The rql-no-hook data importer receives a small amendment but note that: * there exist no test for it * its actual semantics are undefined Hence we cannot prove this hunk breaks nothing, because we cannot prove anything. Closes #3259713.

import ConfigParser
try:
    undo_actions = config.cfgfile_parser.get('MAIN', 'undo-support', False)
except ConfigParser.NoOptionError:
    pass # this conf. file was probably already migrated
else:
    config.global_set_option('undo-enabled', bool(undo_actions))