perl: use mock i18n functions under NO_GETTEXT=Y

perl: use mock i18n functions under NO_GETTEXT=Y

Change the logic of the i18n functions I added in 5e9637c6297 (i18n:
add infrastructure for translating Git with gettext, 2011-11-18) to
use pass-through functions when NO_GETTEXT is defined.

This speeds up the compilation time of commands that use this library
when NO_GETTEXT=Y is in effect. Loading it and POSIX.pm is around 20ms
on my machine, whereas it takes 2ms to just instantiate perl itself.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>