format-patch: respect --stat in cover letter's diffstat

format-patch: respect --stat in cover letter's diffstat

Commit 43662b23ab (format-patch: keep cover-letter diffstat wrapped in
72 columns - 2018-01-24) uncondtionally sets stat width to 72 when
generating diffstat for the cover letter, ignoring --stat from command
line. But it should only do so when stat width is still default
(i.e. stat_width == 0).

In order to fix this, we should only set stat_width if stat_width is
zero. But it will never be. Commit 071dd0ba43 (format-patch: reduce
patch diffstat width to 72 - 2018-02-01) makes sure that default stat
width will be 72 (ignoring $COLUMNS, but could still be overriden by
--stat). So all we need to do here is drop the assignment.

Reported-by: Laszlo Ersek <lersek@redhat.com>
Helped-by: Leif Lindholm <leif.lindholm@linaro.org>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>