Move git operations for multi_action into Gitlab::Git

What does this MR do?

Move git operations for multi_action into Gitlab::Git. This sets the field for the gitaly migration of this feature

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

No

Why was this MR needed?

gitaly#801 (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

What are the relevant issue numbers?

Closes gitaly#872 (closed)