stash: fix nonsense pipeline

An earlier change bba067d2 ("stash: don't delete untracked files
that match pathspec", 2018-01-06) was made by taking a suggestion in
a list discussion [1] but did not copy the suggested snippet
correctly.  And the bug was unnoticed during the review and slipped
through.

This fixes it.

[1] https://public-inbox.org/git/xmqqpo7byjwb.fsf@gitster.mtv.corp.google.com/

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

git-stash.sh

@@ -315,9 +315,9 @@ push_stash () {
 
 		if test $# != 0
 		then
-			git add -u -- "$@" |
-			git checkout-index -z --force --stdin
-			git diff-index -p --cached --binary HEAD -- "$@" | git apply --index -R
+			git add -u -- "$@"
+			git diff-index -p --cached --binary HEAD -- "$@" |
+			git apply --index -R
 		else
 			git reset --hard -q
 		fi