delta-find: assume the target-rev if not specified

This will be useful in the next patch, but I am putting it in its own commit to
make sure we can easily bisect any subtle side effect. (none are expected, but
still)

mercurial/revlogutils/deltas.py

@@ -684,6 +684,9 b' def _candidategroups('
         yield None
         return
 
+    if target_rev is None:
+        target_rev = len(revlog)
+
     if (
         cachedelta is not None
         and nullrev == cachedelta[0]