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/ex_docmd.c b/src/ex_docmd.c
index 97bbd04..c2d6967 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -4293,6 +4293,13 @@
 	    break;
 #endif
 
+	case CMD_argdelete:
+	    while ((xp->xp_pattern = vim_strchr(arg, ' ')) != NULL)
+		arg = xp->xp_pattern + 1;
+	    xp->xp_context = EXPAND_ARGLIST;
+	    xp->xp_pattern = arg;
+	    break;
+
 #endif /* FEAT_CMDL_COMPL */
 
 	default:
@@ -5879,6 +5886,7 @@
     char    *name;
 } command_complete[] =
 {
+    {EXPAND_ARGLIST, "arglist"},
     {EXPAND_AUGROUP, "augroup"},
     {EXPAND_BEHAVE, "behave"},
     {EXPAND_BUFFERS, "buffer"},