fix location revision error for multi parent draft

PLAT-332
@adampalay @dmitchell 
Fix problem where a component has multiple parents (of which all other parents except original parent could be orphans) and when user tires to export the course with draft version of that component then "get_parent_location" method raises "AssertionError" for location revision of that component.
Yeah, I hate the way we do that in old mongo. The idea was that we didn't want any revision information to escape the modulestore in much the same way that we grudgingly decided not to allow branch and version info to escape from split. It makes for ugly machinations to work around this rather than either making the applications branch, revision, version aware or just blind.