Rename `BatchCommitLoader` to `BatchLfsOidLoader`

What does this MR do?

This improves the naming of one of the loaders introduced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28666

Since we're not actually loading commits in the BatchCommitLoader, I now thing BatchLfsOidLoader is a better name.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content