avoid trailing whitespace in zero-change diffstat lines

avoid trailing whitespace in zero-change diffstat lines

In some cases, we produce a diffstat line even though no
lines have changed (e.g., because of an exact rename). In
this case, there is no +/- "graph" after the number of
changed lines. However, we output the space separator
unconditionally, meaning that these lines contained a
trailing space character.

This isn't a huge problem, but in cleaning up the output we
are able to eliminate some trailing whitespace from a test
vector.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>