Past merged MRs do not have approved_approvers for now,
so to keep UI display correctly, compute approved approvers if
association is empty.

Recreate fail case as spec to test creation of `approved_approver`
after merge. Use `inverse_of` to ensure same copy of MR object across
`PostMergeService` and `ApprovalMergeRequestRule`. This means we can
avoid reloading `approval_rules` to solve outdate MR merge state issue.