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.
diff --git a/src/version.c b/src/version.c
index 9311bc5..880a510 100644
--- a/src/version.c
+++ b/src/version.c
@@ -793,6 +793,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    573,
+/**/
     572,
 /**/
     571,