Reviewer roulette is not showing on MR with delete-only files

Problem

When an MR contains only deleted files (no additions or renames) Reviewer Roulette doesn't show up.

Example

Example MR - gitlab-org/gitlab!198391 (closed)

This is because we discard deleted files when trying to attribute files to review categories. (Thanks @leetickett-gitlab for spotting this!)

Proposed solution

If an MR only contains deleted files use these to attribute review categories.

Implementation guide

In changes_by_category use changed and deleted files for categorization:

    def changes_by_category(categories = [])
      changed_and_deleted_files = all_changed_files + changes.deleted.files

      changed_and_deleted_files.each_with_object(Hash.new { |h, k| h[k] = [] }) do |file, hash|
        categories_for_file(file, categories).each { |category| hash[category] << file }
      end
    end