[MERGE][#294479] Fix many round-trips when pushing to pre-1.13 HPSS pack repository

[John Arbash Meinel]

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Andrew Bennetts wrote:
> This fixes two issues:
> 
>  * in bzr.dev, there's a regression vs. 1.13 causing lots of little append round
>    trips when pushing to a pack repo on pre-1.13 smart server.
> 
>  * as a side-effect, this fixes the specific case that has been bothering people
>    for a long time: that pushing to a *stacked* pack repository via pre-1.13
>    HPSS is slow.  That was <https://bugs.launchpad.net/bzr/+bug/294479>.
> 
> In both cases upgrading the server would fix the issue (and give much better
> network performance in general), but within reason we don't want to make life
> too painful for people just because their server is a bit old.
> 
> This patch is a little bit of a hack, but IMO it's not too outrageous.  Thanks
> to Robert for some pointers about where best to place this hack.
> 
> -Andrew.
> 
> 

There aren't any new tests that this is introducing, which seems a bit
odd. At a minimum, shouldn't we have a VF test that asserts that
'flush()' exists?

I suppose it is an Access issue, which does not already a per-XXX
implementation test.

So it seems that something like this should *somehow* generate a new
test. But at a minimum this seems better than what we have, so:

bb:approve

John
=:->
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