patch 8.0.0656: cannot use ! after some user commands

Problem:    Cannot use ! after some user commands.
Solution:   Properly check for existing command. (Higashi Higashi)
diff --git a/src/version.c b/src/version.c
index 533660f..fd57e48 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    656,
+/**/
     655,
 /**/
     654,