Unwanted empty space at end of output

[dcolascione]

In _output_screen_diff we unconditionally draw a character cell (perhaps containing a space) at each line whether or not that line has any actual content. The result is unwanted spaces appearing at the end of output, e.g. if you start ipython in an empty terminal, the bottom-left corner of that terminal will have a space in it.

If we replace

                output_char(new_char)

with

                if new_max_line_len:
                    output_char(new_char)

we don't output the character when we nothing to show and end up with a cleaner terminal for the user.