Include generated ref commits in oldest per commit finder

What does this MR do and why?

This MR adds the functionality to include merge requests via the generated ref commits table for the oldest per commit finder, e.g. used for generating changelogs.

References

Query plan: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/43841/commands/134184

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #568100