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/ex_docmd.c b/src/ex_docmd.c
index 4722edd..387d26d 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -12007,7 +12007,7 @@
 
     if (fputs(cmd, fd) < 0 || put_eol(fd) == FAIL)
 	return FAIL;
-    if (put_line(fd, "silent! argdel *") == FAIL)
+    if (put_line(fd, "%argdel") == FAIL)
 	return FAIL;
     for (i = 0; i < gap->ga_len; ++i)
     {