Fix gitaly ref encoding bugs

What does this MR do?

Adds missing encoding coercion calls on Gitaly ref responses.

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

Why was this MR needed?

We were injecting ASCII-8BIT strings into GitLab, that doesn't work. It has to be UTF-8.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• 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-ce/issues/34156