Fix a bug in $$ escape handling

That caused uninstall and install everytime for a part containing $$ in its
saved options. Also it caused broken .installed.cfg in case of error, like the
referenced section was not defined.

src/zc/buildout/buildout.py

@@ -735,6 +735,7 @@ class Buildout(UserDict.DictMixin):
                                 new_options['__buildout_signature__'] = old_signature
                             else:
                                 del(new_options['__buildout_signature__'])
+                    new_options = new_options.copy()
                     if old_options == new_options:
                         # The options are the same, but are all of the
                         # installed files still there?  If not, we should
@@ -821,9 +822,9 @@ class Buildout(UserDict.DictMixin):
                 saved_options['__buildout_installed__'] = installed_files
                 saved_options['__buildout_signature__'] = signature
                 installed_part_options[part] = saved_options
-                part in installed_parts or installed_parts.append(part)
-
-                _check_for_unused_options_in_section(self, part)
+                if part not in installed_parts:
+                    installed_parts.append(part)
+                    _check_for_unused_options_in_section(self, part)
 
         finally:
             installed_path = self['buildout']['installed']

src/zc/buildout/tests.py

@@ -1021,6 +1021,7 @@ Uninstall recipes need to be called when a part is removed too:
     uninstalling
     Installing demo.
     installing
+    Unused options for demo: 'x'.
 
 
     >>> write('buildout.cfg', '''