x/build/maintner: GitHub issue events are not consistent with API values

Moved from #28226…

There are some issues that have events missing within the maintner corpus. This makes it impossible to create an accurate milestone burndown chart where you want to query for the state of an issue at a particular time window.

A few examples of issues in maintner that have incomplete event lists:

=== Issue events for golang.org/issues/28559
             labeled	milestone:         	label:Testing
             labeled	milestone:         	label:help wanted
             labeled	milestone:         	label:OS-OpenBSD
             labeled	milestone:         	label:Builders
             labeled	milestone:         	label:NeedsInvestigation
          milestoned	milestone:   Go1.12	label:

It does not record the final “closed” event: https://api.github.com/repos/golang/go/issues/28559/events

=== Issue events for golang.org/issues/28306
           mentioned	milestone:         	label:
          subscribed	milestone:         	label:
           mentioned	milestone:         	label:
          subscribed	milestone:         	label:
            assigned	milestone:         	label:
             labeled	milestone:         	label:Documentation
             labeled	milestone:         	label:NeedsInvestigation
          milestoned	milestone:   Go1.12	label:
             renamed	milestone:         	label:

The above event log is missing a few milestone-related events: https://api.github.com/repos/golang/go/issues/28306/events