don't include cases that were referenced by indices in case updates

should fix test failures introduced by https://github.com/dimagi/casexml/pull/231/files

http://manage.dimagi.com/default.asp?121698
https://github.com/dimagi/casexml/pull/231/files#diff-c89da2b94592a4069786651f90753871R242 was populating the cache with index reference cases even if they weren't updated by the form. this was causing doc update conflicts in tests (as well as unnecessarily saving those cases during submissions)