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(-)