Gettextize the "--cached cannot be used with --files" message. Since
this message starts with "--" we have to pass "--" as the first
argument. This works with both GNU gettext 0.18.1 (as expected), and
the gettext(1) on Solaris 10.
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>