Avoid calculation of closes_issues.

## What does this MR do?

Avoid unneeded calls to MR closes issues

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

I'm not sure if calling this method from a view is a good practice, but I cannot see another simple way of avoiding this problem. In case we want to avoid this in the controller we need to specify the action, format and status of the merge request, because in that case we know that the `_open` partial will render. We could add some lazy evaluation but it not a thing I see in use along the app but feedback is welcome

## What are the relevant issue numbers?

#14202 , #19490

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- ~~[ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- ~~[ ] API support added~~
- Tests
  - ~~[ ] Added for this feature/bug~~
  - [x] All builds are passing
- [x] 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 !5140