Don't use rugged write-ref anymore

What does this MR do?

Use shelling out to git to write refs instead of rugged, hoping to avoid creating invalid refs.

To update HEAD we switched to using git symbolic-ref.

What are the relevant issue numbers?

This is part of the solution for https://gitlab.com/gitlab-org/gitlab-ce/issues/44431

It depends on gitaly!982 (merged) & gitaly-proto!241 (merged)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.