Automated Testing: Use three-dot diff comparison for changelog checks

[aduth]

What?

Fixes an issue where the "Check Package Changelogs" CI check may produce false positives.

Related: #79460 (comment)

Why?

Our tests should be accurate to reflect their expected requirements, lest false positive cause confusion or decrease trust in the tests.

How?

Before these changes, the changelog check was effectively doing git diff $BASE_SHA HEAD, which is a "two dot" operation (ref).

For pull request changes, three-dot notation should be used, and is consistent with how GitHub presents pull request changes:

Since the three-dot comparison compares with the merge base, it is focusing on "what a pull request introduces."

For the false positive flagged in #79460, you can see the specific difference in behavior with how the PR's last commit compares against trunk as it exists at the point of the check being run:

• Two dot: https://github.com/WordPress/gutenberg/compare/12358ae0038..0c286d33e8b (includes packages/theme changes which is what triggered the test failure)
• Three dot: 12358ae...0c286d3 (does not include packages/theme changes)

Testing Instructions

Observe that the sample failure commit triggers test failure for "Check changelog" test.

Use of AI Tools

Used Cursor + Composer for research, changes were applied myself.

[github-actions]

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Co-authored-by: aduth <aduth@git.wordpress.org>
Co-authored-by: ciampo <mciampini@git.wordpress.org>
Co-authored-by: jsnajdr <jsnajdr@git.wordpress.org>

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[github-actions]

Size Change: 0 B

Total Size: 7.51 MB

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[ciampo]

🚀

I never really considered this aspect of git diffing.

[aduth]

I never really considered this aspect of git diffing.

Yeah I learned a lot today 😄 Also relevant is that "base SHA" is a moving target in the context of these tests and depends on when the checks are run, where typically we're thinking in terms of the merge-base commit.

[github-actions]

Flaky tests detected in 7d3b4d0.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/28177258096
📝 Reported issues:

• [Flaky Test] should update the HTML only for the latest navigation #78835 in /test/e2e/specs/interactivity/router-navigate.spec.ts

[jsnajdr]

Makes sense 👍