Avoid Gitaly n+1 times per request for Project Branches performance

What does this MR do?

Enables us to reuse root_ref_hash (the commit hash of the default branch).

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

How can I check if there are N+1 Gitaly calls in the CI tests?

Instead of the CI, the following is the test results on my local macOS 10.13 with GDK env:

Before	After
1292.13ms / 44 gitaly	118ms / 5 gitaly

Why was this MR needed?

To avoid N+1 Gitaly queries to improve the page speed.

Does this MR meet the acceptance criteria?

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

What are the relevant issue numbers?

Closes #44657 (closed)

Closes #37429 (closed)

Might help #12619 (moved)