Code review comment for lp:~spiv/bzr/inventory-delta

This is the same patch updated for bzr.dev, and with InterDifferingSerializer restored for local fetches only. For non-local fetches I'm pretty sure the streaming code path is massively faster based on the timings I've done (even via TCP HPSS to localhost).

I'm sure we do want to get rid of IDS eventually (and fix the shortcomings in streaming that John has pointed out) but doing that shouldn't block the rest of this work, even if it is a small maintenance headache.

[FWIW, if I don't restrict IDS to local-only branches I get test failures like:

Traceback (most recent call last):
  File "/home/andrew/warthogs/bzr/inventory-delta/bzrlib/tests/per_interrepository/test_fetch.py", line 137, in test_fetch_parent_inventories_at_stacking_boundary_smart_old
    self.test_fetch_parent_inventories_at_stacking_boundary()
  File "/home/andrew/warthogs/bzr/inventory-delta/bzrlib/tests/per_interrepository/test_fetch.py", line 181, in test_fetch_parent_inventories_at_stacking_boundary
    self.assertCanStreamRevision(unstacked_repo, 'merge')
  File "/home/andrew/warthogs/bzr/inventory-delta/bzrlib/tests/per_interrepository/test_fetch.py", line 187, in assertCanStreamRevision
    for substream_kind, substream in source.get_stream(search):
  File "/home/andrew/warthogs/bzr/inventory-delta/bzrlib/remote.py", line 1895, in missing_parents_chain
    for kind, stream in self._get_stream(sources[0], search):
  File "/home/andrew/warthogs/bzr/inventory-delta/bzrlib/smart/repository.py", line 537, in record_stream
    for bytes in byte_stream:
  File "/home/andrew/warthogs/bzr/inventory-delta/bzrlib/smart/message.py", line 338, in read_streamed_body
    _translate_error(self._body_error_args)
  File "/home/andrew/warthogs/bzr/inventory-delta/bzrlib/smart/message.py", line 361, in _translate_error
    raise errors.ErrorFromSmartServer(error_tuple)
ErrorFromSmartServer: Error received from smart server: ('error', "<bzrlib.groupcompress.GroupCompressVersionedFiles object at 0xb06348c> has no revision ('sha1:98fd3a13366960dc27dcb4b6ddb2b55aca3aae7b',)")

(for scenarios like Pack6RichRoot->2a).]

Anyway, please review.