t7002: set test prerequisite "external-grep" if supported Add another test to set prerequisite EXTGREP if the current build supports external grep. This can be used to skip external grep only tests on builds that do not support this optimization. Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>