Fix line rendering when newline is added at the end of the buffer

[tompng]

Fixed #505

When newline is added at the end of the buffer, reline re-calculates previous line indentation.
Reline needs to render both previous line and added line, but renderes only added line.
I changed rerender_added_newline to also render previous line.

[tompng]

To see which line is re-rendered, I used this snippet paste to bin/console

def (IRB::Color).colorize_code(code,**)
  code.chars.map do
    "\e[#{rand(30..37)}m#{_1}"
  end.join
end

[ima1zumi]

Why did you move this line?

[tompng]

I move it because I am using whole_lines in line 979.

In other place of line_editor.rb, whole_lines is called in this form

if @previous_line_index
  whole_lines(index: @previous_line_index, line: @line)
else
  whole_lines
end

whole_lines also depends on @buffer_of_lines, @line and @line_index. Calling whole_lines between changing these instance variables seems dangerous for me.
After moving it, @buffer_of_lines @line @previous_line_index is changed all at once.

If I don't move it, it still works but won't work after #460 is merged.

[ima1zumi]

I see. I understood. Thank you for your reply.