patch 8.0.1649: no completion for argument list commands

Problem:    No completion for argument list commands.
Solution:   Add arglist completion. (Yegappan Lakshmanan, closes #2706)
diff --git a/src/vim.h b/src/vim.h
index 42007ed..626c0ad 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -781,6 +781,7 @@
 #define EXPAND_PACKADD		45
 #define EXPAND_MESSAGES		46
 #define EXPAND_MAPCLEAR		47
+#define EXPAND_ARGLIST		48
 
 /* Values for exmode_active (0 is no exmode) */
 #define EXMODE_NORMAL		1