Expose merge request diff variables

What does this MR do?

Introduce the following variables:

• CI_MERGE_REQUEST_DIFF_ID
• CI_MERGE_REQUEST_DIFF_BASE_SHA

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation
• Code review guidelines
• Merge request performance guidelines
• Style guides

Availability and Testing

I tested this with detached merge request pipelines and wrote tests for merged results. As this feature is not available in the UI without a license, I would welcome testing of this part.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Closes #273387 (closed)