Move git operations for UpdateRemoteMirrorService into Gitlab::Git

What does this MR do?

Prepare UpdateRemoteMirrorService for migration into Gitaly by moving all git operations into Gitlab::Git.

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

Yes. See https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3824#note_52038012

Why was this MR needed?

Client preparation for gitaly#789 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Closes gitaly#841 (closed)