format-patch: reduce patch diffstat width to 72

format-patch: reduce patch diffstat width to 72

Patches generated by format-patch are meant to be exchanged as emails,
most of the time. And since it's generally agreed that text in mails
should be wrapped around 70 columns or so, make sure these diffstat
follow the convention (especially when used with --cover-letter since we
already defaults to wrapping 72 columns). The default can still be
overriden with command line options.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>