Implement fix for n+1 issue on `flatten_tree` helper

What does this MR do?

Use the new field flat_path that Gitaly populates to skip calculating it when rendering the views, which is an N+1 operation

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

No

Why was this MR needed?

CommitService.GetTreeEntries RPC was being called in a n+1 fashion, which is no good.

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#530 (closed)