Fix errors happening when source branch of merge request is removed and then restored

What does this MR do?

It a follow-up on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7481 and the reasoning is explained there.

Screenshots (if relevant)

Before

After

Does this MR meet the acceptance criteria?

• Changelog entry added
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together