i18n: add--interactive: mark status words for translation

i18n: add--interactive: mark status words for translation

Mark words 'nothing', 'unchanged' and 'binary' used to display what has
been staged or not, in "git add -i" status command.

Alternatively one could mark N__('nothing') no-op in order to
xgettext(1) extract the string and then trigger the translation at run
time only with __($print->{FILE}), but that has the side effect of triggering
retrieval of translations for the changes indicator too (e.g. +2/-1)
which may or may not be a problem.

To avoid that potential problem, mark only where there is certain to
trigger translation only of those words but in this case we must also
retrieve the translation for the eq tests, since the value assigned was
of the translation, not the English source.

Signed-off-by: Vasco Almeida <vascomalmeida@sapo.pt>
Signed-off-by: Junio C Hamano <gitster@pobox.com>