Resolve "Rebase not working with forked projects in 9.4.2"

What does this MR do?

Fix rebasing when the MR is from a fork, and the upstream has the source branch protected (or you just can't push upstream anyway).

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

I can't do a real test for this, because we don't run hooks in the specs, because we'd need to run a server for them to hit the internal API 😞 I have tested it manually, however.

The other way to solve this would be to override project inside the RebaseService, but I think this is clearer.

Why was this MR needed?

We broke something when fixing another bug 🤦

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• 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

What are the relevant issue numbers?

Closes #3071 (closed).