[MERGE] Add BzrDir.get_branch_stacked_on_url

[Aaron Bentley]

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Jonathan Lange wrote:
> Here's a new patch that does this.
> 
> I hope it satisfies.

bb:reject

By using a TransportConfig instead of a BranchConfig, this changes the
behavior of get_stacked_on_url.  The idea was allow stacked_on_url to be
configured in locations.conf or bazaar.conf.

BranchConfig is supposed to do locking through the branch, which
requires a branch instance.  So attempting to access config data without
constructing a branch instance is a flawed concept.

Yet, the rationale for wanting this API is reasonable.  We should be
able to decide which branches we open on a location-by-location basis.
Basically, we are straitjacketed by our existing API.

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.

Personally, I prefer 1, but 2 has some advantages in terms of speed and
simplicity.

Aaron
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