Make git-add -i accept ranges like 7-

Make git-add -i accept ranges like 7-

git-add -i ranges expect number-number. But for the supremely lazy, typing in
that second number when selecting "from patch 7 to the end" is wasted effort.
So treat an empty second number in a range as "until the last item".

Signed-off-by: Ciaran McCreesh <ciaran.mccreesh@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>