fix: use merge-base for accurate change detection in pj-rehearse

[deepsm007]

When a PR is created from an outdated base branch commit, git diff-tree comparisons can be incorrect because they compare against the old base SHA instead of finding the actual merge point.

This fix uses git merge-base to find the common ancestor between the base SHA and HEAD, ensuring accurate change detection even when:

• The base branch has moved forward significantly
• The PR was created from an outdated commit
• Merge commits exist in the PR history

This resolves the issue where pj-rehearse fails to detect changes until the PR is rebased onto the latest base branch.

/cc @danilo-gemoli @openshift/test-platform

/hold

[openshift-ci-robot]

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[coderabbitai]

Walkthrough

This change modifies the getRevChanges function in pkg/config/release.go to calculate and use a merge-base as the reference point for diff computation. When merge-base succeeds, its trimmed result replaces the original base in the diff comparison; on failure, the function falls back to the original base. This refines change detection when the base branch has advanced.

Changes

Cohort / File(s)	Summary
Merge-base calculation for diff refinement pkg/config/release.go	Adds merge-base calculation to determine moving base for diffs in getRevChanges. Implements error-tolerant handling with fallback to original base; trims whitespace from merge-base result before using it as the left-hand side of diff-tree invocation. Preserves existing filtering, diff invocation, and post-processing logic.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

• Verify merge-base error handling and fallback semantics are correctly implemented
• Confirm whitespace trimming logic handles edge cases properly
• Assess impact on change detection accuracy when base branch has advanced
• Ensure the diff-tree command structure is valid with the merge-base substitution

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• Post copyable unit tests in a comment

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📥 Commits

Reviewing files that changed from the base of the PR and between f0f4b3b and 017e948.

📒 Files selected for processing (1)

• pkg/config/release.go (1 hunks)

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Files:

• pkg/config/release.go

🔇 Additional comments (1)

pkg/config/release.go (1)

277-288: LGTM! Robust solution for accurate change detection.

The merge-base approach correctly handles the case where the base branch has advanced since the PR was created. The fallback logic (lines 281-284) ensures safe operation when merge-base fails (e.g., shallow clones, no common ancestor), preserving the original behavior. This makes the implementation more robust than the previous version.

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[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: deepsm007

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[deepsm007]

/test images

[openshift-ci]

@deepsm007: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/breaking-changes	017e948	link	false	/test breaking-changes
ci/prow/images	017e948	link	true	/test images

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[danilo-gemoli]

I'm not sure the code we are changing here is ever exercised by pj-rehearse. If my understanding is correct, this is the relevant part. What do you think?