"Reset approvals on push" does not reset approvals for re-opened MRs

Summary

Reset approvals on push should clear any approvers if a push has been made to the branch. It is currently not.

Steps to reproduce

You need a minimum of 2 approvers, and separate user making and pushing changes to a branch.

1. User A opened a MR from branch X.
2. User B approved it.
3. User A closed MR.
4. User A pushed a commit to branch X.
5. User A reopened the MR.

Note that User B's approval is still there, even though changes have happened to the branch after approval.

Expected behavior

User B's approval should be removed even if the branch was closed.

Gitlab version 8.14.5-ee 1ce744fa