Display `parent_ids` in merge request commits API response

What does this MR do and why?

Contributes to #496665

Problem

Merge request commits are stored in the database without information about their parent_ids. As a result, commits created from DB records are missing this data.

Solution

Load commit content from Gitaly to have access to full commit information.

References

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• parent_ids field always empty list in merge req... (#496665)

MR acceptance checklist

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Screenshots or screen recordings

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Before	After

How to set up and validate locally

1. Enable feature flag

   Feature.enable(:commits_from_gitaly)

2. Visit merge request commits API endpoint (http://gdk.test:3000/api/v4/projects/462/merge_requests/1/commits)

3. parent_ids value should be defined for commits (when FF is disabled, it will be missing)

4. Find a merge request diff version from this endpoint (http://gdk.test:3000/api/v4/projects/462/merge_requests/1/versions/)

5. Visit merge request diff version page (http://gdk.test:3000/api/v4/projects/462/merge_requests/1/versions/2180)

6. parent_ids value should be defined for commits (when FF is disabled, it will be missing. Note endpoint response is cached)

Related to #496665