lib/backup: fix broken permissions when creating repo dir

## What does this MR do?

This MR fixes an issue where gitlab:backup:restore will not work because the repositories directory is set up with bad permissions. The bad permissions will prevent access to the repositories by the git user, causing all kinds of troubles (e.g. gitlab-shell won't install hooks).

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

## Why was this MR needed?

Rake task gitlab:backup:restore would complete successfully, but gitlab:check will report errors and gitlab cannot access repos.

## Screenshots (if relevant)

* repositories.old.1472478726 was created without the fix
* repositories was created with the fix

![gitlab-repo-permissions](/uploads/6cf2de57fe4f1be0bd5b5e1a13234889/gitlab-repo-permissions.png)

## 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
  - [ ] 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)

## What are the relevant issue numbers?

See merge request !6098