Add ref selector to commit list header

What does this MR do and why?

This is part of a bigger effort to refactor the repository Commit list. All changes are behind the FF project_commits_refactor`. Specifically, this MR adds the ref selector to the commit list header. This matches exactly the existing ref selector when FF is off.

References

Finish state of the header

Implementation Plan

Task	MR
Add ref selector	👈 this MR
Add breadcrumb	follow up
Add overflow menu	!200595 (merged)
Add filter search and support author search	!199810 (merged)
Add support for message search	!200595 (merged)

Screenshots or screen recordings

FF off	FF on

CleanShot 2025-08-12 at 15.43.09.mp4

How to set up and validate locally

1. Enable project_commits_refactor FF
2. Open a project
3. Code > Commits
4. Select branch/tag/hash from the ref selector

⚠️ the list of commits uses mock data and would not change when ref gets updated. to validate this change, i confirm the link generated from the new ref elector matches the existing one (when FF is off) 👍

MR acceptance checklist

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Related to #551400 (closed)