delta-find: move the emotion of prev in a dedicated method

After splitting the filtering, and with the `_candidate_groups` layer removed,
we can start splitting the group generation too. This helps to organize this
code and make it easier to modifying the future.

mercurial/revlogutils/deltas.py

@@ -1074,6 +1074,12 b' class _DeltaSearch(_BaseDeltaSearch):'
             # Test all parents (1 or 2), and keep the best candidate
             yield parents
 
+    def _iter_prev(self):
+        # other approach failed try against prev to hopefully save us a
+        # fulltext.
+        self.current_stage = _STAGE_PREV
+        yield (self.target_rev - 1,)
+
     def _refined_groups(self):
         good = None
         groups = self._raw_groups()
@@ -1221,10 +1227,7 b' class _DeltaSearch(_BaseDeltaSearch):'
             yield tuple(sorted(full))
 
         if not sparse:
-            # other approach failed try against prev to hopefully save us a
-            # fulltext.
-            self.current_stage = _STAGE_PREV
-            yield (prev,)
+            yield from self._iter_prev()
 
 
 class SnapshotCache: