Prepare cherry-pick and revert for migration to Gitaly

What does this MR do?

Move those parts of Repository#cherry_pick and Repository#revert that touch Git into Gitlab::Git.

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

Why was this MR needed?

All code that touches Git must move to Gitaly. We are doing this via Gitlab::Git.

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
• 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

What are the relevant issue numbers?