Backport of 'Skip the post-merge diff commit preload in RefreshService' (19.3)

What does this MR do and why?

Backport and enable the RefreshService fix that skips preloading the latest diff commit metadata graph for all open MRs targeting a pushed branch, before candidate filtering.

Backport rationale, rollback, and verification

• Candidate mitigation for high CPU on a GitLab Dedicated tenant; the tenant's root cause is not yet confirmed.
• Two clean cherry-pick -x commits independently target 19-3-stable-ee, based on fresh origin/19-3-stable-ee at 97e1a8d8b6e2 (19.3.3-ee). GitLab 19.3 lacks this code; both refresh fixes were first verified shipped default-on in 19.4.0. The original preload fix deployed to production on August 24, and its default-on change on September 8.
• merge_request_refresh_skip_diff_commit_preload is a project-scoped beta flag with default_enabled: true. Disable it for the project to restore the old path.
• git diff --check passed; each commit's git patch-id matches upstream. RuboCop passed for both changed Ruby files.

References

• Issue: #618580 (closed)
• Original fix: !251113 (merged) — e0f49df533f3 → fb722ac3d23f.
• Default-on change: !253951 (merged) — ab35d7b271d5 → e486a9f998db.
• Related: #628868 (closed) and https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/team/-/work_items/13987

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.