Compare tree hashes instead of using git diff

[IanButterworth]

Comparing tree hashes is more accurate for detecting content differences between the merge commit and PR head, avoiding false positives when the content is identical but commit SHAs differ.

Fixes #153

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (013f11f) to head (2e2177d).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #154   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines            2         2           
=========================================
  Hits             2         2           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Copilot]

Pull request overview

This PR improves the detection of content differences between the merge commit and PR head in GitHub Actions by comparing tree hashes instead of using git diff. This approach is more accurate as it avoids false positives when content is identical but commit SHAs differ.

Key changes:

• Added a check to verify HEAD^2 exists before attempting to compare commits
• Replaced git diff --quiet with tree hash comparison using git rev-parse
• Added explicit handling for non-merge commits

---

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