Fix: Duplicate options of permitted_labels.

For a given LABEL, e.g. named Qualified, When
the SubmitRecord.Label value is like
'NOT_READY[Qualified: OK, Code-Review: NEED, Qualified: MAY]'
the Qualified options will show duplicated options on WebUI.

Reason: ListMultimap is used to keep permitted_labels and
its values. Fixed by replacing ListMultimap by SetMultimap.
Please note this change only affects permitted_labels.

Change-Id: I8c9683474650233cd131236e78281e67967c2a12
1 file changed