patch 8.2.2991: Vim9: no completion for :vim9 and :legacy

Problem:    Vim9: no completion for :vim9 and :legacy.
Solution:   Expand argument as a command. (closes #8377)
diff --git a/src/cmdexpand.c b/src/cmdexpand.c
index c5b8f70..ba31e92 100644
--- a/src/cmdexpand.c
+++ b/src/cmdexpand.c
@@ -1370,6 +1370,8 @@
 	case CMD_verbose:
 	case CMD_vertical:
 	case CMD_windo:
+	case CMD_vim9cmd:
+	case CMD_legacy:
 	    return arg;
 
 	case CMD_filter: