The `official` flag of a pull request review is computed against the
target branch's protection rules at submit time and stored on the review
record. `ChangeTargetBranch` updated the base branch but never
re-evaluated it, so an `official` approval obtained against an
unprotected branch could be retargeted onto a protected branch and
satisfy its required approvals, bypassing the branch protection.
This re-evaluates the `official` flag of the latest approve/reject
reviews against the new base branch whenever the target branch changes.
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