Filter.OtherGroup: Use a Set for performance
Filter.OtherGroup: Use a Set for performance

This should run faster. In addition, it seemed that the other group's
IDs were concatted unconditionally, so there would be a lot of
duplicates. Random performance cleanup.

https://phabricator.endlessm.com/T22103

endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib