Compute PR diff-ranges during init

[kaspersv]

When diff-informed analysis is performed, PR diff-ranges are currently computed by the analyze action. This PR moves computation and persistence of PR diff-ranges to the init action instead. The motivation for moving the PR diff-range computation to the init action is to make it available for overlay analysis and include all files within the PR diff-range in the overlay-changes.json file generated for overlay analysis during the init action.

The commits have been cherry-picked from #3192 and modified.

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?

• Advanced setup - Impacts users who have custom workflows.
• Default setup - Impacts users who use default setup.
• Code Scanning - Impacts Code Scanning (i.e. analysis-kinds: code-scanning).
• Code Quality - Impacts Code Quality (i.e. analysis-kinds: code-quality).
• GHES - Impacts GitHub Enterprise Server.

How did/will you validate this change?

• Test repository - This change will be tested on a test repository before merging.
• Unit tests - I am depending on unit test coverage (i.e. tests in .test.ts files).

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

• Feature flags - All new or changed code paths can be fully disabled with corresponding feature flags.
• Rollback - Change can only be disabled by rolling back the release or releasing a new version with a fix.

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

• Telemetry - I rely on existing telemetry or have made changes to the telemetry.
  • Dashboards - I will watch relevant dashboards for issues after the release. Consider whether this requires this change to be released at a particular time rather than as part of a regular release.

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.