FileUtils#compare_stream

# File tmp/rubies/ruby-3.1.3/lib/fileutils.rb, line 827
def compare_stream(a, b)
  bsize = fu_stream_blksize(a, b)

  sa = String.new(capacity: bsize)
  sb = String.new(capacity: bsize)

  begin
    a.read(bsize, sa)
    b.read(bsize, sb)
    return true if sa.empty? && sb.empty?
  end while sa == sb
  false
end