patch 8.1.0821: xxd "usage" output and other arguments not tested

Problem:    Xxd "usage" output and other arguments not tested.
Solution:   Add a test to trigger the usage output in various ways.  Fix
            uncovered problem.
diff --git a/src/version.c b/src/version.c
index e999fd1..ad2f5e7 100644
--- a/src/version.c
+++ b/src/version.c
@@ -788,6 +788,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    821,
+/**/
     820,
 /**/
     819,