Fix a wrong MR status when merge_when_build_succeeds & project.only_allow_merge_if_build_succeeds are true

## What does this MR do?

Fix a wrong MR status when merge_when_build_succeeds & project.only_allow_merge_if_build_succeeds are true.

## Are there points in the code the reviewer needs to double check?

@stanhu I reused your proposal from the issue, I think it's a good enough solution.

## What are the relevant issue numbers?

Fixes #19035.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !4912