[MERGE] Add BzrDir.get_branch_stacked_on_url

[Aaron Bentley]

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Aaron Bentley wrote:
> There are some options:
> 1. Lazy-load stacked_on branches.  We have not reached consensus that
>    this is the right idea yet, and progress may be slow.
> 2. Provide a way (e.g. a flag on Branch.open) to open a branch without
>    opening its stacked_on branch
> 3. Rework our API so that BranchConfig doesn't require a Branch
>    instance.

Forgot one:
4. deliberately change behavior so that only branch.conf is used when
determining get_stacked_on_url.  It can be argued that this is necessary
information for using this branch, so it should be stored with the rest
of the branch data, not in bazaar.conf or locations.conf.

Aaron
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