WIP: Batch call experiment

What does this MR do?

An experiment in batch-calling Gitaly.

Uses the https://github.com/exAspArk/batch-loader Batch-Loader gem.

Watch the presentation, here: https://speakerdeck.com/exaspark/batching-a-powerful-way-to-solve-n-plus-1-queries

Read the blog post: Batching – A powerful way to solve N+1 queries every Rubyist should know

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

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

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