delta-find: introduce and use specialized _DeltaSearch class

For now, we introduce some very simple variant, but they are still useful to
display how having the class can helps keeping the simple case simple and
their special case out of more advanced logic.

mercurial/revlogutils/deltas.py

@@ -783,8 +783,43 b' class _BaseDeltaSearch(abc.ABC):'
         pass
 
 
+class _NoDeltaSearch(_BaseDeltaSearch):
+    """Search for no delta.
+
+    This search variant is to be used in case where we should not store delta.
+    """
+
+    def _init_group(self):
+        pass
+
+    def next_group(self, good_delta=None):
+        pass
+
+
+class _PrevDeltaSearch(_BaseDeltaSearch):
+    """Search for delta against the previous revision only
+
+    This search variant is to be used when the format does not allow for delta
+    against arbitrary bases.
+    """
+
+    def _init_group(self):
+        self.current_group = [self.target_rev - 1]
+
+    def next_group(self, good_delta=None):
+        self.current_group = None
+
+
 class _DeltaSearch(_BaseDeltaSearch):
+    """Generic delta search variants
+
+    (expect this to be split further)
+    """
+
     def _init_group(self):
+        # Why search for delta base if we cannot use a delta base ?
+        # also see issue6056
+        assert self.revlog.delta_config.general_delta
         self._candidates_iterator = self._candidate_groups()
         self._last_good = None
         self.current_group = self._candidates_iterator.send(self._last_good)
@@ -801,17 +836,6 b' class _DeltaSearch(_BaseDeltaSearch):'
         worthwhile content. See _raw_candidate_groups for details about the
         group order.
         """
-        # should we try to build a delta?
-        if not (len(self.revlog) and self.revlog._storedeltachains):
-            yield None
-            return
-
-        if not self.revlog.delta_config.general_delta:
-            # before general delta, there is only one possible delta base
-            yield (self.target_rev - 1,)
-            yield None
-            return
-
         good = None
 
         group_chunk_size = self.revlog.delta_config.candidate_group_chunk_size
@@ -1062,9 +1086,6 b' class _DeltaSearch(_BaseDeltaSearch):'
 
         The group order aims at providing fast or small candidates first.
         """
-        # Why search for delta base if we cannot use a delta base ?
-        assert self.revlog.delta_config.general_delta
-        # also see issue6056
         sparse = self.revlog.delta_config.sparse_revlog
         prev = self.target_rev - 1
         deltachain = lambda rev: self.revlog._deltachain(rev)[0]
@@ -1548,7 +1569,16 b' class deltacomputer:'
             msg %= target_rev
             self._write_debug(msg)
 
-        search = _DeltaSearch(
+        # should we try to build a delta?
+        if not (len(self.revlog) and self.revlog._storedeltachains):
+            search_cls = _NoDeltaSearch
+        elif not self.revlog.delta_config.general_delta:
+            # before general delta, there is only one possible delta base
+            search_cls = _PrevDeltaSearch
+        else:
+            search_cls = _DeltaSearch
+
+        search = search_cls(
             self.revlog,
             revinfo,
             p1r,