Resolve "Very poor performance creating merge request, regression in 9.0 - often hits 60 seconds timeout on small requests"

What does this MR do?

1. Speed up suggested approvers by only shelling out to git once.
2. Don't include the current user in the suggestions when there is no MR author.

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

🤷

Why was this MR needed?

When MRs had a lot of files, and the packfiles were large, MR creation could be slow.

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 #1811 (closed).