Filter Bugs

- A substitution arg is categorized as "subst", but was tested for
  "substitution".
- The Arguments::is_multipart method was triggering on '/', which is
  wrong, but I can't remember why I put it in there.  Hmmm.
- When extracting a write-filter, substitutions after the command
  were generating an error, whereas they should be ignored.
- Task::substitute was modifying data even when the patterns were not
  matching.  Yikes.
- Added unit tests for en-passant modification of all types during
  done, delete, start, stop.

src/Task.cpp

@@ -861,8 +861,11 @@ void Task::substitute (
     }
   }
 
-  set ("description", description);
-  setAnnotations (annotations);
+  if (changes)
+  {
+    set ("description", description);
+    setAnnotations (annotations);
+  }
 }
 
 ////////////////////////////////////////////////////////////////////////////////