git-submodule: abort if core.worktree could not be set correctly

74d4731da1 (submodule--helper: replace connect-gitdir-workingtree by
ensure-core-worktree, 2018-08-13) forgot to exit the submodule operation
if the helper could not ensure that core.worktree is set correctly.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -548,7 +548,7 @@ cmd_update()
 	do
 		die_if_unmatched "$quickabort" "$sha1"
 
-		git submodule--helper ensure-core-worktree "$sm_path"
+		git submodule--helper ensure-core-worktree "$sm_path" || exit 1
 
 		update_module=$(git submodule--helper update-module-mode $just_cloned "$sm_path" $update)