deltas: accept and skip None return for delta info

They are some extra computation that will shortcut the delta compression if the
delta seems hopeless, returning None.

mercurial/revlogutils/deltas.py

@@ -1025,8 +1025,9 b' class deltacomputer(object):'
                 nominateddeltas.append(deltainfo)
             for candidaterev in candidaterevs:
                 candidatedelta = self._builddeltainfo(revinfo, candidaterev, fh)
-                if isgooddeltainfo(self.revlog, candidatedelta, revinfo):
-                    nominateddeltas.append(candidatedelta)
+                if candidatedelta is not None:
+                    if isgooddeltainfo(self.revlog, candidatedelta, revinfo):
+                        nominateddeltas.append(candidatedelta)
             if nominateddeltas:
                 deltainfo = min(nominateddeltas, key=lambda x: x.deltalen)
             if deltainfo is not None: