git-stash apply --index: optimize postprocessing

git-stash apply --index: optimize postprocessing

Originally, "apply --index" codepath was bolted on to the
"update working tree files and index, but then revert the
changes we make to the index except for added files so that we
do not forget about them" codepath, almost as an afterthought.
Because "apply --index" first prepares the final index state
upfront, "revert except the added paths" postprocessing does not
have to be done.

Signed-off-by: Junio C Hamano <gitster@pobox.com>