delta-find: move tested in the _DeltaSearch.__init__

Now that we have an object we can initialize that attribute at initialization
time. This will make it available for more method in the future, allowing to
split the code.

mercurial/revlogutils/deltas.py

@@ -715,6 +715,8 b' class _DeltaSearch:'
             snapshot_cache = SnapshotCache()
         self.snapshot_cache = snapshot_cache
 
+        self.tested = {nullrev}
+
     def candidate_groups(self):
         """Provides group of revision to be tested as delta base
 
@@ -741,7 +743,7 b' class _DeltaSearch:'
         deltas_limit = self.textlen * LIMIT_DELTA2TEXT
         group_chunk_size = self.revlog.delta_config.candidate_group_chunk_size
 
-        tested = {nullrev}
+        tested = self.tested  # prefetch for speed and code compactness
         candidates = self._refined_groups()
         while True:
             temptative = candidates.send(good)