Submit a single batch blob RPC to Gitaly per HTTP request when viewing diffs

What does this MR do?

Submit a single batch blob RPC to Gitaly per Gitlab::Diff::FileCollection::MergeRequestDiff#diff_files method call. This currently affects the DiffsController#show performance.

Locally, it improves the request time in ~15%, let's see it on production.

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/43951