Code review comment for lp:~spiv/bzr/hpss-cross-format-fetch-bug-427736

This fixes "unknown object type identifier 60" in bencode when pulling some branches.

It turns out that the client-side RemoteStreamSource was using the wrong serialiser to decode revision fulltexts in a stream of a stacked branch (it decodes them to extract the parent_ids so that it knows if there are any missing revisions that it needs to ask the next fallback for). Using the wrong serialiser doesn't matter if the revision serialiser is the same on both sides, and similarly this code doesn't get involved when there is no stacking. So this was just obscure enough a case that the test suite wasn't exercising it.

The fix is a one-liner, and I've added interrepo tests for this case.