britney: Leave upgrade_output alone with --dry-run

Closes: https://bugs.debian.org/763521
Signed-off-by: Niels Thykier <niels@thykier.net>

britney.py

@@ -2824,7 +2824,10 @@ class Britney(object):
             self.upgrade_me = self.options.actions.split()
 
         if self.options.compute_migrations or self.options.hint_tester:
-            if hasattr(self.options, 'upgrade_output'):
+            if self.options.dry_run:
+                self.logger.info("Upgrade output not (also) written to a separate file"
+                                 " as this is a dry-run.")
+            elif hasattr(self.options, 'upgrade_output'):
                 upgrade_output = getattr(self.options, 'upgrade_output')
                 file_handler = logging.FileHandler(upgrade_output, mode='w', encoding='utf-8')
                 output_formatter = logging.Formatter('%(message)s')