delta-find: move target_rev in the _DeltaSearch.__init__

Now that we have an object we can initialize that attribute at initialization
time.

mercurial/revlogutils/deltas.py

@@ -699,6 +699,8 b' class _DeltaSearch:'
         self.p2 = p2
         self.cachedelta = cachedelta
         self.excluded_bases = excluded_bases
+        if target_rev is None:
+            self.target_rev = len(self.revlog)
         self.target_rev = target_rev
         if snapshot_cache is None:
             # map: base-rev: [snapshot-revs]
@@ -717,9 +719,6 b' class _DeltaSearch:'
             yield None
             return
 
-        if self.target_rev is None:
-            self.target_rev = len(self.revlog)
-
         if not self.revlog.delta_config.general_delta:
             # before general delta, there is only one possible delta base
             yield (self.target_rev - 1,)