[MERGE] various tweaks to push

[Michael Hudson]

Michael Hudson wrote:
> Robert Collins wrote:
>> This is a few tweaks all conflated, because the issues weren't clear.
>>
>> Basically - we were churning locks a lot (known), and code depended on
>> the locks being released for some methods (bad). 
> 
> Super glad to see this being worked on!
> 
> I filed some related bugs a while ago:
> 
> https://bugs.edge.launchpad.net/bzr/+bug/283457
> "set_stacked_on_location doesn't take a write lock"
> 
> https://bugs.edge.launchpad.net/bzr/+bug/283479
> "set_stacked_on_location leaves a locked repossitory with an unlocked
> fallback repository"
> 
> I think they can both be closed when this branch lands.

I see a later version of this patch landed, so I closed the bugs.

Cheers,
mwh