Use commit finder instead of rev parse

What does this MR do?

Use a different way to fetch the commit data.

Why was this MR needed?

Makes the method free of rugged calls.

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] API support added
• [-] Tests added for this feature/bug
• Review
  • [-] Has been reviewed by UX
  • [-] Has been reviewed by Frontend
  • Has been reviewed by Backend
  • [-] Has been reviewed by Database
• [-] Conform by the merge request performance guides
• [-] Conform by the style guides
• [-] Squashed related commits together
• [-] Internationalization required/considered

What are the relevant issue numbers?

Spotted this was happening on: gitaly#743 (comment 49090905)