[MERGE] Streaming fetch from remote servers.

[Andrew Bennetts]

Robert Collins wrote:
> Use our shiny network streaming code for fetching too, in the
> non-stacked source case. Removes 66% of the round trips needed to stream
> from a short(9 commits) branch to a new local branch.
> 
> Also included, a test that we can branch remotely (this is something
> that could otherwise be broken by a casual change to reuse transports
> more aggressively in branch, for instance).

bb:approve

> @@ -3508,8 +3371,6 @@
>  InterRepository.register_optimiser(InterSameDataRepository)
>  InterRepository.register_optimiser(InterWeaveRepo)
>  InterRepository.register_optimiser(InterKnitRepo)
> -InterRepository.register_optimiser(InterModel1and2)
> -InterRepository.register_optimiser(InterKnit1and2)
>  InterRepository.register_optimiser(InterPackRepo)
>  InterRepository.register_optimiser(InterOtherToRemote)
>  InterRepository.register_optimiser(InterRemoteToOther)

This change is particularly heart-warming!

-Andrew.