Use merge-base for git audit comparisons

[MikeMcQuaid]

• compare brew audit changes to the branch point, not contemporary origin/HEAD commits
• avoid false positives when unrelated dependency bumps land after a branch is cut
• cover changed_formulae_paths and committed_version_info with merge-base regression specs

Fixes #21861

---

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests (excluding integration tests) for your changes? Here's an example.
• Have you successfully run brew lgtm (style, typechecking and tests) with your changes locally?

---

• AI was used to generate or assist with generating this PR. Please specify below how you used AI to help you, and what steps you have taken to manually verify the changes.

Used OpenAI Codex with manual review and feedback.

---

[Copilot]

Pull request overview

Updates brew audit’s git-based comparisons to use the merge-base between origin/HEAD and HEAD, so audits evaluate changes relative to the branch point rather than the moving tip of origin/HEAD (avoiding false positives from unrelated upstream commits).

Changes:

• Switch changed_formulae_paths to diff from a merge-base SHA (with fallback to origin/HEAD).
• Switch committed_version_info history traversal to start from the merge-base SHA.
• Add regression specs covering merge-base behavior for both paths and version-history logic.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
Library/Homebrew/formula_auditor.rb	Introduces git_audit_base_ref and uses it for git diff/history lookups instead of origin/HEAD.
Library/Homebrew/test/formula_auditor_spec.rb	Adds/adjusts specs to assert merge-base is used for diffs and history traversal.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.