Return a dict with fields that differ between two dicts.

def _update_on_fields(prev_ele, new_ele):
    '''
    Return a dict with fields that differ between two dicts.
    '''
    fields_update = dict(
        (prop_name, prop_val)
        for prop_name, prop_val in six.iteritems(new_ele)
        if new_ele.get(prop_name) != prev_ele.get(prop_name) or prop_name in _MATCH_KEYS
    )
    if len(set(fields_update.keys()) | set(_MATCH_KEYS)) > len(set(_MATCH_KEYS)):
        if 'id' not in fields_update:
            # in case of update, the ID is necessary
            # if not specified in the pillar,
            # will try to get it from the prev_ele
            fields_update['id'] = prev_ele['id']
        return fields_update