DPTP-4632: 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When a PR is created from an outdated base branch commit, git diff-tree comparisons can be incorrect because they compare against the old base SHA instead of finding the actual merge point.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• The PR was created from an outdated commit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]

Pipeline controller notification
This repo is configured to use the pipeline controller. Second-stage tests will be triggered either automatically or after lgtm label is added, depending on the repository configuration. The pipeline controller will automatically detect which contexts are required and will utilize /test Prow commands to trigger the second stage.

For optional jobs, comment /test ? to see a list of all defined jobs. To trigger manually all jobs from second stage use /pipeline required command.

This repository is configured in: automatic mode

[coderabbitai]

Walkthrough

Compute and use a merge-base in getRevChanges (pkg/config/release.go) for diffs (with fallback), tighten error reporting in prepareCandidate to return the rebase error, and expand the empty-job message in getJobsTableLines (cmd/pj-rehearse/server.go).

Changes

Cohort / File(s)	Summary
Merge-base calculation for diff refinement pkg/config/release.go	getRevChanges now computes a merge-base between base and HEAD, trims it, and uses mergeBase:path as the left-hand side for git diff-tree; on merge-base failure it falls back to the original base. Existing ignore/filtering and post-processing remain.
Rebase error propagation cmd/pj-rehearse/server.go	prepareCandidate returns the actual rebaseErr when rebasing fails instead of a generic err, improving error specificity.
Empty jobs message expansion cmd/pj-rehearse/server.go	getJobsTableLines expands the returned message when there are no presubmits or periodics to include guidance about possible rebasing and step-registry changes, and adds extra line breaks.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

✨ Finishing touches

• 📝 Generate docstrings

---

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[deepsm007]

/test images

[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?

[openshift-ci-robot]

@deepsm007: This pull request references DPTP-4632 which is a valid jira issue.

Warning: The referenced jira issue has an invalid target version for the target branch this PR targets: expected the story to target the "4.22.0" version, but no target version was set.

Details

In response to this:

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

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[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: deepsm007, Prucek

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Details

Needs approval from an approver in each of these files:

• OWNERS [Prucek,deepsm007]

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[deepsm007]

/unhold

[deepsm007]

/test images

[openshift-ci-robot]

Scheduling required tests:
/test e2e

Scheduling tests matching the pipeline_run_if_changed or not excluded by pipeline_skip_if_only_changed parameters:
/test integration-optional-test

[deepsm007]

/override ci/prow/integration-optional-test

[openshift-ci]

@deepsm007: Overrode contexts on behalf of deepsm007: ci/prow/integration-optional-test

Details

In response to this:

/override ci/prow/integration-optional-test

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[openshift-ci]

@deepsm007: The following test 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	1d15dd1	link	false	/test breaking-changes

Full PR test history. Your PR dashboard.

Details

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