As per the docs (and relevant docstring), users who can access private repositories can also access its private review requests. This works fine, until an invite-only review group is added to the review request. This patch fixes and tests for the bug.
Access Control docs:
Review Request Access Summary
To summarize, a user has access to a review request only if all the following conditions are met:
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 * The repository is public or the user has access to to it (either by being explicitly on the user access list, or by being a member of a group on that list).
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PS. I recommend reviewing the raw or expanded diff, as reviewboard has made the visual diff below very jumbled

Test cases in patch pass.