Merge branch 'ak/t4151-ls-files-could-be-empty' into maint

Test fix.

* ak/t4151-ls-files-could-be-empty:
  t4151: make sure argument to 'test -z' is given

t/t4151-am-abort.sh

@@ -82,7 +82,7 @@ test_expect_success 'am -3 --abort removes otherfile-4' '
 	test 4 = "$(cat otherfile-4)" &&
 	git am --abort &&
 	test_cmp_rev initial HEAD &&
-	test -z $(git ls-files -u) &&
+	test -z "$(git ls-files -u)" &&
 	test_path_is_missing otherfile-4
 '