Refactoring after moving fast-forward merge to CE

What does this MR do?

We made fast-forward merge CE feature so that means that some code on EE side should be relocated and changed.

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

Yeah, maybe you know some other place that should be changed too.

Why was this MR needed?

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?

https://gitlab.com/gitlab-org/gitlab-ce/issues/20076