Move all rugged operation for ff_merge inside Gitlab::Git

What does this MR do?

Move all rugged operation for ff_merge inside Gitlab::Git.

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

No.

Why was this MR needed?

It sets us up to migrate this feature to Gitaly. See gitaly#680 (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

What are the relevant issue numbers?

Closes gitaly#683 (closed)