t4014-format-patch: do not assume 'test' is available as non-builtin

One test case used 'xargs test', which assumes that 'test' is available
as external program. At least on MinGW it is not.

Moreover, 'git format-patch' was invoked in a pipeline, but not as the
last command. Rewrite the test case to catch breakage in 'git format-patch'
as well.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4014-format-patch.sh

@@ -549,9 +549,7 @@ test_expect_success 'options no longer allowed for format-patch' '
 	test_cmp expect.check output'
 
 test_expect_success 'format-patch --numstat should produce a patch' '
-	git format-patch --numstat --stdout master..side |
-	grep "^diff --git a/" |
-	wc -l |
-	xargs test 6 = '
+	git format-patch --numstat --stdout master..side > output &&
+	test 6 = $(grep "^diff --git a/" output | wc -l)'
 
 test_done