t4151: make sure argument to 'test -z' is given

88d50724 (am --skip: revert changes introduced by failed 3way merge,
2015-06-06), unlike all the other patches in the series, forgot to
quote the output from "$(git ls-files -u)" when using it as the
argument to "test -z", leading to a syntax error on platforms whose
test does not interpret "test -z" (no other arguments) as testing if
a string "-z" is the null string (which GNU test and test that is
built into bash and dash seem to do).

Note that $(git ls-files -u | wc -l) is deliberately left unquoted,
as some implementations of "wc -l" includes extra blank characters
in its output and cannot be compared as string, i.e. "test 0 = $(...)".

Signed-off-by: Armin Kunaschik <megabreit@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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
 '