tests/test-diffstat.out

diff: add --stat for diffstat output

diff/qdiff --stat invokes patch.diffstat() on the diff output.

When in interactive mode, the output's maximum width is determined by the
terminal's width.

% wide diffstat
 a |  213 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 213 insertions(+), 0 deletions(-)
% diffstat width
 a |  213 ++++++++++++++
 1 files changed, 213 insertions(+), 0 deletions(-)
% narrow diffstat
 a |  3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
% binary diffstat
 b |  0 
 1 files changed, 0 insertions(+), 0 deletions(-)