Fix commit list date grouping to use committedDate

What does this MR do and why?

Fixes date grouping on the commit list page to use committedDate instead of authoredDate.

References

• Reported in #598401 (comment 3448592697)
• #598401 (comment 3451736529)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the new commit list experience (if behind a feature flag)
2. Navigate to a repository with merge commits where authoredDate and committedDate differ (e.g., a repo with long-lived feature branches merged out of chronological author order)
3. Visit the commits page (/<project>/-/commits/<branch>)
4. Before: date headers would appear out of order (e.g., "9 Jun" → "10 Jun" → "9 Jun")
5. After: date headers should flow in monotonically decreasing order matching the commit stream

🗒️ here is the list of steps i did in a test project to achieve the screenshots above

git checkout -b branch-old 
git commit --allow-empty -m "Commit on old base" --date="2026-06-08T10:00:00"

git checkout master -b branch-new
git commit --allow-empty -m "Commit on new base" --date="2026-06-15T10:00:00"

git checkout -b test-merge-history master 
git merge branch-old 
git merge branch-new

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