Avoid /dev/null on the command line.

The shell turns it into 'nul' (which we can't understand as an existing
file).

t/t4116-apply-reverse.sh

@@ -42,7 +42,8 @@ test_expect_success 'apply in reverse' '
 	git reset --hard second &&
 	git apply --reverse --binary --index patch &&
 	git diff >diff &&
-	git diff /dev/null diff
+	: > empty &&
+	git diff empty diff
 
 '