Fetch branch divergence through one method call

What does this MR do?

Consolidates branch divergence queries to one method call returning a hash in preparation for one Gitaly call to retrieve the information as merged branches now do.

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

N/A

Why was this MR needed?

Prepare for Gitaly migration to improve performance.

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
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #37429 (closed)