Revert diffstat back to English

Revert diffstat back to English

This reverts the i18n part of 7f81463 (Use correct grammar in diffstat
summary line - 2012-02-01) but still keeps the grammar correctness for
English. It also reverts b354f11 (Fix tests under GETTEXT_POISON on
diffstat - 2012-08-27). The result is diffstat always in English
for all commands.

This helps stop users from accidentally sending localized
format-patch'd patches.

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