Add "--patch" option to git-add--interactive

Add "--patch" option to git-add--interactive

When the "--patch" option is supplied, the patch_update_cmd() function is
called bypassing the main_loop() and exits.

Seeing as builtin-add is the only caller of git-add--interactive we can
impose a strict requirement on the format of the arguments to avoid
possible ambiguity: an "--" argument must be used whenever any pathspecs
are passed, both with the "--patch" option and without it.

Signed-off-by: Wincent Colaiuta <win@wincent.com>