[MERGE] Add support for branch-associated locations

[Ian Clatworthy]

Aaron Bentley wrote:
> Hi all,
> 
> This patch adds support for branch-based location lookups.
> Specifically, the values ':bound', ':parent', ':public', ':push',
> ':this' and ':submit'.

I like this. Some ultra minors tweak to consider ...

bb:tweak

> +    :parent, :submit, :public, :push, :this,and :bound are currently

You don't need the comma after :this. If you want to keep it, add a
space after it.

> +class DirectoryLookupFailure(BzrError):
> +    """Base type for lookup errors."""
> +
> +    pass

I'm pretty sure that the docstring is enough as the class body, i.e.
you don't need pass here. OTOH, pass does make it explicit that you
meant the class to be empty so I'm fine with leaving it there if
you prefer. I'm not sure what PEP-8 says in this case, if anything.
Your call IMO.

Ian C.