Refactor Gitlab::Git::Commit to include a repository and add missing specs

What does this MR do?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/37000

Refactor Gitlab::Git::Commit to include a repository and add missing specs

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

No

Why was this MR needed?

We were getting production errors when Gitaly commits were being asked for their rugged commit counterpart, because of an incomplete migration of the changes included in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13094

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/37000