Include child team members in collaborators
Include child team members in collaborators

GitHub's Nested Teams feature makes it possible to put teams inside
other teams. The regular v3 API does not include child team members when
listing collaborators, which leads to missing permissions when Bors runs
a reviewer sync.

As described in the [Collaborator API docs][1], setting the
`hellcat-preview` content type will cause the API to include members of
child teams when listing contributors.

[1]: https://developer.github.com/v3/repos/collaborators/

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