patch 8.1.0573: cannot redefine user command without ! in same script

Problem:    Cannot redefine user command without ! in same script
Solution:   Allow redefining user command without ! in same script, like with
            functions.

src/version.c

@@ -792,6 +792,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    573,
 /**/
     572,
 /**/