Support doing migration-less runs

Signed-off-by: Niels Thykier <niels@thykier.net>

britney.py

@@ -409,6 +409,10 @@ class Britney(object):
                                help="just print a summary of uninstallable packages")
         parser.add_option("", "--components", action="store", dest="components",
                                help="Sources/Packages are laid out by components listed (, sep)")
+        parser.add_option("", "--compute-migrations", action="store_true", dest="compute_migrations", default=True,
+                          help="Compute which packages can migrate (the default)")
+        parser.add_option("", "--no-compute-migrations", action="store_false", dest="compute_migrations",
+                          help="Do not compute which packages can migrate.")
         (self.options, self.args) = parser.parse_args()
         
         # integrity checks
@@ -2736,6 +2740,7 @@ class Britney(object):
         else:
             self.upgrade_me = self.options.actions.split()
 
+        if self.options.compute_migrations or self.options.hint_tester:
             with open(self.options.upgrade_output, 'w', encoding='utf-8') as f:
                 self.__output = f
 
@@ -2749,6 +2754,8 @@ class Britney(object):
                 self.log('> Stats from the installability tester', type="I")
                 for stat in self._inst_tester.stats.stats():
                     self.log('>   %s' % stat, type="I")
+        else:
+            self.log('Migration computation skipped as requested.', type='I')
 
 
 if __name__ == '__main__':