Don't load blob or content_commits on Gitlab::Diff::File instance for discussions

What does this MR do?

To avoid git calls

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

Why was this MR needed?

What are the relevant issue numbers?

Relates with #14202 (moved) but there are more work todo

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

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