Refactor nuninst initialization to avoid reading the cache from disk

At the moment, britney will always read the nuncache from disk
(possibly regenerating it before reading it).  However, we can save
the "read from disk" by moving the initialization of nuninst into the
constructor of Britney and relying on clone_nuninst.

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

britney.py

@@ -357,6 +357,7 @@ class Britney(object):
             self.logger.info("Building the list of non-installable packages for the full archive")
             self._inst_tester.compute_testing_installability()
             nuninst = self.get_nuninst(build=True)
+            self.nuninst_orig = nuninst
             for arch in self.options.architectures:
                 self.logger.info("> Found %d non-installable packages", len(nuninst[arch]))
                 if self.options.print_uninst:
@@ -376,6 +377,12 @@ class Britney(object):
                 self.logger.info(">  %s", arch)
                 for stat in arch_stat.stat_summary():
                     self.logger.info(">  - %s", stat)
+        else:
+            self.logger.info("Loading uninstallability counters from cache")
+            self.nuninst_orig = self.get_nuninst()
+
+        # nuninst_orig may get updated during the upgrade process
+        self.nuninst_orig_save = clone_nuninst(self.nuninst_orig, architectures=self.options.architectures)
 
         for policy in self.policies:
             policy.hints = self.hints
@@ -2449,11 +2456,6 @@ class Britney(object):
         output_logger.info("Generated on: %s", time.strftime("%Y.%m.%d %H:%M:%S %z", time.gmtime(time.time())))
         output_logger.info("Arch order is: %s", ", ".join(self.options.architectures))
 
-        self.logger.info("> Calculating current uninstallability counters")
-        self.nuninst_orig = self.get_nuninst()
-        # nuninst_orig may get updated during the upgrade process
-        self.nuninst_orig_save = self.get_nuninst()
-
         if not self.options.actions:
             # process `easy' hints
             for x in self.hints['easy']:
@@ -2584,10 +2586,6 @@ class Britney(object):
         This method provides a command line interface for the release team to
         try hints and evaluate the results.
         """
-        self.logger.info("> Calculating current uninstallability counters")
-        self.nuninst_orig = self.get_nuninst()
-        self.nuninst_orig_save = self.get_nuninst()
-
         import readline
         from britney2.completer import Completer