patch 8.1.0789: sourcing a session sets v:errmsg

Problem:    Sourcing a session sets v:errmsg.
Solution:   Use "%argdel" instead of "argdel *". (Jason Franklin)
diff --git a/src/version.c b/src/version.c
index fb1e356..3647397 100644
--- a/src/version.c
+++ b/src/version.c
@@ -792,6 +792,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    789,
+/**/
     788,
 /**/
     787,