Store all modified tasks for use by on-exit hook (#3352)

The on-exit hook gets all modified tasks as input, but this was omitted
in the previous release. This adds a test for the desired behavior, and
updates TDB2 to correctly store the required information.

src/TDB2.h

@@ -84,6 +84,9 @@ private:
   tc::Replica replica;
   std::optional<tc::WorkingSet> _working_set;
 
+  // UUID -> Task containing all tasks modified in this invocation.
+  std::map<std::string, Task> changes;
+
   const tc::WorkingSet &working_set ();
   static std::string option_string (std::string input);
   static void show_diff (const std::string&, const std::string&, const std::string&);