git add: add long equivalents of '-u' and '-f' options

git add: add long equivalents of '-u' and '-f' options

The option -u stands for --update and it is a good idea to make it clear
especially because this is the only mode of operation of "git add" that
does something different from "adding".  Give longer --force synonym to -f
while we are at it as well.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>