[Buildbot] #3273: GerritChangeFilter support for branch filtering is broken

[Buildbot trac]

#3273: GerritChangeFilter support for branch filtering is broken
-------------------+-----------------------
Reporter:  sa2ajj  |      Owner:
    Type:  defect  |     Status:  new
Priority:  major   |  Milestone:  undecided
 Version:  0.8.12  |   Keywords:  gerrit
-------------------+-----------------------
 Basically, current implementation expects every event coming from Gerrit
 to have a change included.  As [https://gerrit-
 documentation.storage.googleapis.com/Documentation/2.11/cmd-stream-
 events.html#events documentation] shows, there're some events that do not
 have any changes included, most notablbe `ref-updated`.

 As a practical example, a set of properties for an event with a change:
 {{{
 event.change.branch
 event.change.commitMessage
 event.change.id
 event.change.number
 event.change.owner.email
 event.change.owner.name
 event.change.owner.username
 event.change.project
 event.change.status
 event.change.subject
 event.change.topic
 event.change.url
 event.patchSet.author.email
 event.patchSet.author.name
 event.patchSet.author.username
 event.patchSet.createdOn
 event.patchSet.isDraft
 event.patchSet.number
 event.patchSet.parents
 event.patchSet.ref
 event.patchSet.revision
 event.patchSet.sizeDeletions
 event.patchSet.sizeInsertions
 event.patchSet.uploader.email
 event.patchSet.uploader.name
 event.patchSet.uploader.username
 event.type
 event.uploader.email
 event.uploader.name
 event.uploader.username
 }}}

 and here's a set of properties for an event that does not have a change
 (`ref-updated`):
 {{{
 event.refUpdate.newRev
 event.refUpdate.oldRev
 event.refUpdate.project
 event.refUpdate.refName
 event.submitter.email
 event.submitter.name
 event.submitter.username
 event.type
 }}}

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Ticket URL: <http://trac.buildbot.net/ticket/3273>
Buildbot <http://buildbot.net/>
Buildbot: build/test automation