Enrich commits with full data in CommitCollection

What does this MR do?

CommitCollection now has a method #enrich which replaces non-gitaly commits with commits from gitaly.

This allows collections of commits that have intentionally incomplete data (e.g., from PostgreSQL) to convert to full records for parts of the app that need all the data.

CommitCollection#without_merge_commits has been updated to call this method, as in order to determine a merge commit we need to have parent data.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/58805

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

Closes #58805 (closed)