Ensure Rugged methods are called with a Rugged Commit

What does this MR do?

Ensure Rugged methods are called with a Rugged::Commit. In 9.6, these changes were already brought by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13094, so they're only needed in 9.5.

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

No

Why was this MR needed?

Some Gitaly features will make Gitaly::Commit the raw commit object, which will result in errors for methods that are expecting a Rugged::Commit as the raw object.

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?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/36718