Use --left-right and --max-count for counting diverging commits

What does this MR do?

Use --left-right and --max-count for counting diverging commits

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

Gitaly doesn't support this yet (see gitaly#382 (comment 48993695)), so it should have no effect for ~performance for Gitaly (but we're still showing 999+ if it's over 1000, to keep consistent behaviour), but if we're not using Gitaly it should boost some ~performance.

Why was this MR needed?

Trying to solve https://gitlab.com/gitlab-org/gitlab-ce/issues/40622

Screenshot

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Backend

What are the relevant issue numbers?

This might not completely resolve #40622 (closed) yet