Fix importing with an invalid MR source project

Source project shouldn't be -1 when both source and target are the same in the original export.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/20611

• CHANGELOG entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together