Don't produce v2 chunks with a null delta except the first one

While Mercurial handles those chunks correctly, it wouldn't produce them
(and git-cinnabar didn't handle them well before the previous commit).

cinnabar/hg/bundle.py

@@ -593,7 +593,7 @@ def prepare_chunk(store, chunk, previous, chunk_type):
  key=len)
  if len(deltas):
  return deltas[0]
- return chunk.to_chunk(chunk_type)
+ return chunk.to_chunk(chunk_type, previous)
  else:
  assert False