diff --stat: allow custom diffstat output width.

diff --stat: allow custom diffstat output width.

This adds two parameters to "diff --stat".

 . --stat-width=72 tells that the page should fit on 72-column output.

 . --stat-name-width=30 tells that the filename part is limited
   to 30 columns.

Signed-off-by: Junio C Hamano <redacted>