fix cascading failure with the index conversion code

this resolves bug #something where the index file could not be
converted, completely breaking conversion.

it seems that, during some refactoring, the index conversion code was
completely dropped. this was missed by the unit tests because the repo
can still be opened by the constructor even though the index is
invalid, so tests need improvements there.

borg/upgrader.py

@@ -139,12 +139,14 @@ class AtticRepositoryUpgrader(Repository):
           `Cache.open()`, edit in place and then `Cache.close()` to
           make sure we have locking right
         """
-        caches = []
         transaction_id = self.get_index_transaction_id()
         if transaction_id is None:
             logger.warning('no index file found for repository %s' % self.path)
         else:
-            caches += [os.path.join(self.path, 'index.%d' % transaction_id).encode('utf-8')]
+            cache = os.path.join(self.path, 'index.%d' % transaction_id).encode('utf-8')
+            logger.info("converting index cache %s" % cache)
+            if not dryrun:
+                AtticRepositoryUpgrader.header_replace(cache, b'ATTICIDX', b'BORG_IDX')
 
         # copy of attic's get_cache_dir()
         attic_cache_dir = os.environ.get('ATTIC_CACHE_DIR',