test: use test_i18ncmp when checking --stat output

test: use test_i18ncmp when checking --stat output

Ever since v1.7.9.2~13 (2012-02-01), git's diffstat-style summary line
produced by "git apply --stat", "git diff --stat", and "git commit"
varies by locale, producing test failures when GETTEXT_POISON is set.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>