Fix gerrit leave comment capability

When adding support for short build_uuid's in
AI6356a971ca250ddf5f01a9734f13d0b080a62c89 event.bugs was converted to a
set since we would know can run classify multiple times on a singe
event and don't want duplicate bugs. That patch didn't update the gerrit
leave comment capabilties to understand event.bugs as a set (instead of
a list).

Change-Id: I9032e23e0e53426a57bebf42f4c4d4167624280e