Cache commits on the repository model

To be perfectly honest, caching based on branch name or other refs I found a bit scary still. But IMO this would be a good start.

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

What are the relevant issue numbers?

gitaly#658 (closed)