vim-patch:8.1.2348: :const cannot be followed by "| endif"

Problem:    :const cannot be followed by "| endif".
Solution:   Check following command for :const. (closes vim/vim#5269)
            Also fix completion after :const.
8f76e6b12b

src/nvim/ex_docmd.c

@@ -2126,6 +2126,7 @@ static char_u * do_one_cmd(char_u **cmdlinep,
     case CMD_browse:
     case CMD_call:
     case CMD_confirm:
+    case CMD_const:
     case CMD_delfunction:
     case CMD_djump:
     case CMD_dlist:
@@ -3437,6 +3438,7 @@ const char * set_one_cmd_context(
   case CMD_syntax:
     set_context_in_syntax_cmd(xp, arg);
     break;
+  case CMD_const:
   case CMD_let:
   case CMD_if:
   case CMD_elseif: