vim-patch:8.0.1649: no completion for argument list commands

Problem:    No completion for argument list commands.
Solution:   Add arglist completion. (Yegappan Lakshmanan, closes vim/vim#2706)
cd43effeca

src/nvim/vim.h

@@ -156,6 +156,7 @@ enum {
   EXPAND_PACKADD,
   EXPAND_MESSAGES,
   EXPAND_MAPCLEAR,
+  EXPAND_ARGLIST,
   EXPAND_CHECKHEALTH,
 };