[MERGE] Add BzrDir.get_branch_stacked_on_url

[Aaron Bentley]

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Robert Collins wrote:
> By the same token, we don't strictly need a Repository object to be open
> either when we open a tree or branch, but we settled a while ago, I
> thought, that it was safer and better to do so. I think the same
> arguments apply here.

I'm pretty sure I don't agree.  Could you repeat the arguments?

To me, there is a significant difference, because a stacked branch that
can't access its stacked-on branch can still support many, many
operations.  Lack of a Repository, on the other hand, is crippling.

> A stacked branch where the stacked on Repository
> is no longer accessible should be raised as a problem as soon as
> possible to the user.

I'd rather optimize for success than failure.  I think we should open
stacked branches as quickly as normal branches, which means not hitting
the network to open stacked-on branches, since they may not be needed.

I think if an operation can complete locally without using external
resources, then it should do so.

That's how shallow branches will work.  Or are you saying that shallow
branches are a bad idea?

Aaron
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