Show all todo items when printing in Dot format

By doing so, the parent items, which are hidden by default, are shown in
the dependency graph. Otherwise it would only print child items which is
not so useful.

topydo/commands/ListCommand.py

@@ -62,6 +62,10 @@ class ListCommand(ExpressionCommand):
                     if self._poke_icalendar():
                         from topydo.lib.IcalPrinter import IcalPrinter
                         self.printer = IcalPrinter(self.todolist)
+
+                        # a graph without dependencies is not so useful, hence
+                        # show all
+                        self.show_all = True
                 elif value == 'dot':
                     from topydo.lib.DotPrinter import DotPrinter
                     self.printer = DotPrinter(self.todolist)