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

@@ -702,7 +702,7 @@ class Buildout(DictMixin):
             if part in install_parts:
                 old_options = self.installed_part_options[part].copy()
                 installed_files = old_options.pop('__buildout_installed__')
-                new_options = self.get(part)
+                new_options = self.get(part).copy()
                 if not self.check_signature:
                     old_signature = old_options.get(
                         '__buildout_signature__', None)
@@ -957,7 +957,7 @@ class Buildout(DictMixin):
                         for k, v in _spacey_defaults:
                             value = value.replace(k, v)
                         options[option] = value
-                result[section] = self.Options(self, section, options)
+                result[section] = self.Options(self, section, options).copy()
 
             return result
         else:

src/zc/buildout/buildout.txt

@@ -2050,6 +2050,7 @@ changes, the service shouldn't be changed.
     >>> print_(system(buildout), end='')
     Develop: '/sample-buildout/recipes'
     Updating service.
+    Unused options for service: 'script'.
 
 Now we change the service part to trigger uninstallation and
 re-installation.

src/zc/buildout/tests.py

@@ -1007,6 +1007,7 @@ Uninstall recipes need to be called when a part is removed too:
     uninstalling
     Installing demo.
     installing
+    Unused options for demo: 'x'.
 
 
     >>> write('buildout.cfg', '''