TW-1580: "modified" attribute not updated

- Make all local modifications update the "modified" attribute.
- As per design, the user cannot overwrite this attribute; neither
  can hooks.

ChangeLog

@@ -2,6 +2,7 @@
 
 - TW-1578 Bash tab completion problems on first run
           (thanks to Renato Alves and Ptolemarch).
+- TW-1580 "modified" attribute no longer updated (thanks to David Patrick).
 - Setting 'bulk' to zero is interpreted as infinity, which means there is no
   amount of changes that is considered dangerous (thanks to Tomas Babej).
 

src/TDB2.cpp

@@ -598,7 +598,7 @@ void TDB2::update (
   const std::string& uuid,
   Task& task,
   const bool add_to_backlog,
-  const bool addition)
+  const bool addition /* = false */)
 {
   // Validate to add metadata.
   task.validate (false);
@@ -607,6 +607,13 @@ void TDB2::update (
   Task original;
   if (not addition && get (task.get ("uuid"), original))
   {
+    if (add_to_backlog)
+    {
+      // All locally modified tasks are timestamped, implicitly overwriting any
+      // changes the user or hooks tried to apply to the "modified" attribute.
+      task.setAsNow ("modified");
+    }
+
     // Update the task, wherever it is.
     if (!pending.modify_task (task))
       completed.modify_task (task);