[MERGE] Add BzrDir.get_branch_stacked_on_url

[Aaron Bentley]

Jonathan Lange wrote:
> On Tue, Sep 2, 2008 at 9:58 AM, Aaron Bentley <aaron at aaronbentley.com> wrote:
>> Aaron Bentley has voted comment.
>> Status is now: Waiting
>> Comment:
>> Why?  Does Branch.open hit the stacked_on_url?
>>
> 
> Yes, it does.

Well, I'm not opposed to this as a stop-gap as long as you file a bug
that we shouldn't open fallback branches until we know that we need them.

The patch as it stands assumes too much about the way branches are
configured.  This would fail spectacularly with a Subversion branch, if
they supported stacking.

There should be indirection through the branch format to ensure that
branch formats can determine how their configuration is acquired and how
stacking in particular is configured.  That code should be reused by
BzrBranch6 in the implementation of get_stacked_on_url.

bb:resubmit

Aaron