```ruby
require_relative 'hunk_base'
require_relative 'hunk'

module GitCrecord
  module Hunks
    class File < HunkBase
      attr_reader :hunks

      def initialize(filename_a, filename_b)
        @filename_a = filename_a
        @filename_b = filename_b
        @hunks = []
        @extra_lines = []
        @expanded = false
        super()
      end

      def to_s
        @filename_a == @filename_b ? @filename_a : "#{filename_a} -> #{filename_b}"
      end

      def info_string
        line_count = subs.reduce(0){ |a, e| e.highlightable_subs.size + a }
        "  #{subs.size} hunk(s), #{line_count} line(s) changed"
      end

      def strings(width)
        result = to_s.scan(/.{1,#{width}}/)
        return result unless expanded
        result += info_string.scan(/.{1,#{width}}/)
        result << ''
      end

      def <<(hunk)
        @hunks << Hunk.new(hunk)
      end

      def add_hunk_line(line)
        @hunks.last << line
      end

      def add_extra_line(line)
        @extra_lines << line
      end

      def subs
        @hunks
      end

      def highlightable_subs
        @hunks
      end
    end
  end
end
```

Implement generate_diff method for FileHunk
user
```ruby
require_relative 'hunk_base'
require_relative 'hunk'

module GitCrecord
  module Hunks
    class File < HunkBase
      attr_reader :hunks

      def initialize(filename_a, filename_b)
        @filename_a = filename_a
        @filename_b = filename_b
        @hunks = []
        @extra_lines = []
        @expanded = false
        super()
      end

      def to_s
        @filename_a == @filename_b ? @filename_a : "#{filename_a} -> #{filename_b}"
      end

      def info_string
        line_count = subs.reduce(0){ |a, e| e.highlightable_subs.size + a }
        "  #{subs.size} hunk(s), #{line_count} line(s) changed"
      end

      def strings(width)
        result = to_s.scan(/.{1,#{width}}/)
        return result unless expanded
        result += info_string.scan(/.{1,#{width}}/)
        result << ''
      end

      def <<(hunk)
        @hunks << Hunk.new(hunk)
      end

      def add_hunk_line(line)
        @hunks.last << line
      end

      def add_extra_line(line)
        @extra_lines << line
      end

      def subs
        @hunks
      end

      def highlightable_subs
        @hunks
      end

      def generate_diff
        return unless selected
        out_line_offset = 0
        [
          "diff --git a/#{@filename_a} b/#{@filename_b}",
          *@extra_lines,
          *subs.map do |sub|
            content, out_line_offset = sub.generate_diff(out_line_offset)
            content
          end.compact
        ].join("\n")
      end
    end
  end
end
```
assistant