i18n: git-stash "unknown option" message

i18n: git-stash "unknown option" message

Gettextize the "unknown option for 'stash save'" message that's shown
on:

    $ git stash save --blah-blah
    error: unknown option for 'stash save': --blah-blah
           To provide a message, use git stash save -- '--blah-blah'
    Usage: git stash list [<options>]

In a translation the second line should be aligned with the first
one. I've added a TRANSLATORS comment to indicate this.

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