Keep track of the name of the modified code when applying fixers

We can then execute the following query, for example, to find out the
bits of code for which there is no information traced at runtime:

(This is an example for the "division" problem)

SELECT filename
FROM modified
WHERE fixer = "division"
AND filename NOT IN
  (SELECT filename
   FROM division)

my2to3/trace.py

@@ -44,17 +44,22 @@ def get_fixers():
   ]
 
 
+trace_modified = create_table("modified", "fixer", "filename")
 def apply_fixers(string, name):
   # This function is inspired by refactoring_tool.refactor_file
 
-  refactoring_tool = RefactoringTool(fixer_names=get_fixers())
   encoding = tokenize.detect_encoding(lambda: string)[0]
+  string = string.decode(encoding) + u"\n" # '\n' to silence certain parse errors
+
+  for fixer in get_fixers():
+    refactoring_tool = RefactoringTool(fixer_names=[fixer])
+    tree = refactoring_tool.refactor_string(string, name)
+    if tree.was_changed:
+      trace_modified(fixer[17:-6], name)
+      string = unicode(tree)
 
-  string += "\n" # Silence certain parse errors
-  s = string.decode(encoding)
-  s = refactoring_tool.refactor_string(s, name)
   # The [:-1] is to take off the \n we added earlier
-  return unicode(s)[:-1].encode(encoding)
+  return string[:-1].encode(encoding)
 
 
 init_py = '__init__' + os.extsep + 'py'