Bug #1022

- Fixed bug #1022, where dependencies were note released when a blocking task
  was completed (thanks to Arkady Grudzinsky).
- The Task object now caches ::is_blocked and ::is_blocking Booleans that are
  determined on pending.data load.
- Simplified and sped up color rule processing using cached values, reducing
  the number of map lookups, and removed loop invariants when the rules are
  not defined.
- Simplified urgency calculations given the cached values for blocked/blocking.
- On load, pending.data is scanned for accurate blocked/blocking status
  determination.
- Obsoleted and removed complex single-task dependency calculations.
- Sped up 'nag' processing by using cached values..
- Modified the 'show' command to consider color.blocking to be valid.
- Added default config value for color.blocking, and included it in the
  precedence list ahead of blocked, as it is more important.
- Updated taskrc.5 man page to include the new color.blocking rule, and its
  place in the rule precedence.

src/TDB2.h

@@ -66,15 +66,20 @@ public:
   int id (const std::string&);
 
   void has_ids ();
+  void auto_dep_scan ();
   void clear ();
   const std::string dump ();
 
+private:
+  void dependency_scan ();
+
 public:
   bool _read_only;
   bool _dirty;
   bool _loaded_tasks;
   bool _loaded_lines;
   bool _has_ids;
+  bool _auto_dep_scan;
   std::vector <Task> _tasks;
   std::vector <Task> _added_tasks;
   std::vector <Task> _modified_tasks;