in verbose mode, save .installed.cfg each time a part is installed.

src/zc/buildout/buildout.py

@@ -834,6 +834,9 @@ class Buildout(DictMixin):
             self.installed_part_options['buildout']['parts'] = (
                 ' '.join(installed_parts))
 
+            if self._log_level < logging.INFO:
+                self._save_installed_options()
+
         self.installed_part_options['buildout']['parts'] = (
             ' '.join(installed_parts))