fixup! Write .installed.cfg only once, in safe way and only if there's any change.

src/zc/buildout/buildout.py

@@ -789,6 +789,14 @@ class Buildout(DictMixin):
                     else:
                         installed_files = list(installed_files)
 
+                    need_to_save_installed = [
+                        p for p in installed_files
+                        if p not in old_installed_files]
+
+                    if need_to_save_installed:
+                        installed_files = (old_installed_files
+                                           + need_to_save_installed)
+
             else: # install
                 __doing__ = 'Installing %s.', part
                 self._logger.info(*__doing__)