git-sh-setup.sh: use "git rev-parse --show-cdup" to check for SUBDIRECTORY_OK

"git rev-parse --git-dir" trick does not play well with worktree

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-sh-setup.sh

@@ -110,7 +110,7 @@ esac
 if [ -z "$SUBDIRECTORY_OK" ]
 then
 	: ${GIT_DIR=.git}
-	GIT_DIR=$(GIT_DIR="$GIT_DIR" git rev-parse --git-dir) || {
+	test -z "$(git rev-parse --show-cdup)" || {
 		exit=$?
 		echo >&2 "You need to run this command from the toplevel of the working tree."
 		exit $exit