vim-patch:8.2.3629: command completion in cmdline window uses global commands

Problem:    Command completion in cmdline window uses global user commands,
            not local commands for the window where it was opened from.
Solution:   Use local commands. (closes vim/vim#9168)
a119812437

src/nvim/ex_getln.c

@@ -6587,6 +6587,13 @@ static int open_cmdwin(void)
   return cmdwin_result;
 }
 
+/// @return true if in the cmdwin, not editing the command line.
+bool is_in_cmdwin(void)
+  FUNC_ATTR_PURE
+{
+  return cmdwin_type != 0 && get_cmdline_type() == NUL;
+}
+
 /// Get script string
 ///
 /// Used for commands which accept either `:command script` or