Fix access checks for rebasing an MR

What does this MR do?

When checking whether to show the rebase button, we (correctly) see if you can push to the source branch.

The controller, however, was checking if you could merge to the target branch, which is a totally different thing!

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

Don't think so.

Why was this MR needed?

This has never worked properly https://gitlab.com/gitlab-org/gitlab-ee/issues/2891#note_34616824

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 https://gitlab.com/gitlab-org/gitlab-ee/issues/2891.

@smcgivern Thanks, looks good to me!

approved this merge request

enabled an automatic merge when the pipeline for 334d4b1c succeeds

merged

mentioned in commit b2a8a27b