GET "projects/:id/repository/commits" endpoint improvements

• Builds Link header correctly by passing commits total_count to Kaminari
• Use pagination indexes starting by 1 (not 0)

---

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Related !5298 (closed) !8985 (closed)

Closes #1381 (closed), #20207 (closed)