Align GitHub diff report with BitBucket Server diff report changes

Overview

Changes made in !815 (merged) should positively affect the GitHub diff report.

Some of them are:

• Single character per row html report
• Missing "Project Migration Results" diff header in each diff_report_*.html file

Tasks

• Update docs if needed (workflow, runbooks, README, migration metrics, etc.)

Acceptance Criteria

• Existing UTs pass locally
• 100% code coverage on new code

@gitlab-org/professional-services-automation/tools/migration