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.
% no bookmarks
no bookmarks set
% set bookmark X
% list bookmarks
* X -1:000000000000
% update to bookmark X
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
% list bookmarks
* X -1:000000000000
% rename
% list bookmarks
* Z -1:000000000000
% new bookmark Y
% list bookmarks
* Y -1:000000000000
Z -1:000000000000
% commit
% list bookmarks
* Y 0:719295282060
Z -1:000000000000
% delete bookmarks
% list bookmarks
no bookmarks set
% update to tip
0 files updated, 0 files merged, 0 files removed, 0 files unresolved