Fix crash when completing a child item after a 'dep add'

When doing a 'dep add' command, the parent lookup dictionary was not kept up to
date. Therefore there was an edge in the dependengy graph that did not exist in
the parent dict. When the child item is completed, then its dependency ID is
looked up, but it doesn't exist, resulting in a crash.

By adding the _add_edge method, it should be easier to keep the edges of the
graph and the content of the dictionary in sync.

topydo/lib/TodoList.py

@@ -99,12 +99,12 @@ class TodoList(TodoListBase):
             for dep in \
                     [dep for dep in self._todos if dep.has_tag('p', dep_id)]:
 
-                self._depgraph.add_edge(hash(p_todo), hash(dep), dep_id)
+                self._add_edge(p_todo, dep, dep_id)
 
         for dep_id in p_todo.tag_values('p'):
             try:
                 parent = self._parentdict[dep_id]
-                self._depgraph.add_edge(hash(parent), hash(p_todo), dep_id)
+                self._add_edge(parent, p_todo, dep_id)
             except KeyError:
                 pass
 
@@ -145,6 +145,10 @@ class TodoList(TodoListBase):
             # todo item couldn't be found, ignore
             pass
 
+    def _add_edge(self, p_from_todo, p_to_todo, p_dep_id):
+        self._parentdict[p_dep_id] = p_from_todo
+        self._depgraph.add_edge(hash(p_from_todo), hash(p_to_todo), p_dep_id)
+
     @_needs_dependencies
     def add_dependency(self, p_from_todo, p_to_todo):
         """ Adds a dependency from task 1 to task 2. """
@@ -200,7 +204,7 @@ class TodoList(TodoListBase):
                 p_from_todo.set_tag('id', dep_id)
 
             p_to_todo.add_tag('p', dep_id)
-            self._depgraph.add_edge(hash(p_from_todo), hash(p_to_todo), dep_id)
+            self._add_edge(p_from_todo, p_to_todo, dep_id)
             append_projects_to_subtodo()
             append_contexts_to_subtodo()
             self.dirty = True