Fix merge conflict handling in the rebuild workflow - #4191

The rebuild workflow is meant to continue when merging the base branch only conflicts in lib, since rebuilding regenerates it. Since #3568, any conflict has failed the merge step instead, for example in this run. Steps run with bash -e, so a conflicting git merge ended the step before its exit code was recorded. The step now runs git merge as an if condition, which -e doesn't apply to, and continues only if MERGE_HEAD shows the merge is still in progress.

Fixing that alone would have exposed a second bug: under pipefail, the check for conflicts outside lib never triggered, because git diff --check exits non-zero whenever it finds a conflict marker, including in lib. The check now lists unmerged paths with git diff --name-only --diff-filter=U, which also catches conflicts without markers, such as modify/delete conflicts.

Risk assessment

For internal use only. Please select the risk level of this change:

• Low risk: Changes are fully under feature flags, or have been fully tested and validated in pre-production environments and are highly observable, or are documentation or test only.

Which use cases does this change impact?

Environments:

• Testing/None - This change does not impact any CodeQL workflows in production.

How did/will you validate this change?

• Other - I ran the merge step's script from the workflow under bash --noprofile --norc -e -o pipefail against scratch repositories. A clean merge passes, conflicts only in lib continue and set merge-in-progress, and content or modify/delete conflicts in src fail and list the file. I also ran the whole workflow on a temporary branch with the same conflict in lib/entry-points.js as the linked run. It succeeded, and the merge commit it pushed had no conflict markers.

If something goes wrong after this change is released, what are the mitigation and rollback strategies?

• Development/testing only - This change cannot cause any failures in production.

How will you know if something goes wrong after this change is released?

• Other - This change only affects the Rebuild Action workflow, so problems will show up as failed or incorrect rebuild runs.

Are there any special considerations for merging or releasing this change?

• No special considerations - This change can be merged at any time.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.