Use menu behavior when completing change-directory

This makes "cd<space><ret>" change to the first completion,
not to $HOME.  This might be surprising but it should make sense.
I don't have a concrete motivation but this should save a Tab press
in some scenarios.

src/commands.cc

@@ -2555,7 +2555,8 @@ const CommandDesc change_directory_cmd = {
              return { 0_byte, cursor_pos,
                       complete_filename(prefix,
                                         context.options()["ignored_files"].get<Regex>(),
-                                        cursor_pos, FilenameFlags::OnlyDirectories) };
+                                        cursor_pos, FilenameFlags::OnlyDirectories),
+                      Completions::Flags::Menu };
         }),
     [](const ParametersParser& parser, Context&, const ShellContext&)
     {