t0021: write "OUT <size>" only on success

t0021: write "OUT <size>" only on success

"rot13-filter.pl" always writes "OUT <size>" to the debug log at the end
of a response.

This works perfectly for the existing responses "abort", "error", and
"success". A new response "delayed", that will be introduced in a
subsequent patch, accepts the input without giving the filtered result
right away. At this point we cannot know the size of the response.
Therefore, we do not write "OUT <size>" for "delayed" responses.

To simplify the code we do not write "OUT <size>" for "abort" and
"error" responses either as their size is always zero.

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>