[rfc] more control over activating fallback repositories (aka what gets stacked on)

[Andrew Bennetts]

Michael Hudson wrote:
> Currently, opening a branch also opens any stacked on branch specified
> in the branch's branch.conf file.  This is inconvenient some of the
> time, particularly in the innards of the codehosting system on Launchpad
> where we want to be able to inspect and maybe interfere with the stacked
> on URL before opening what it points at.
> 
> A reasonable way to do this seems to be to add a
> "dont_activate_fallbacks" (or similarly named) to Branch.open and
> related functions (and then our code would be responsible for calling
> add_fallback_repository with appropriate arguments.
> 
> Does this make sense as an idea?  I'm not completely sure on all the
> places that would have to have arguments added to push the
> dont_activate_fallbacks argument all the way to where it's needed, but
> that's just typing I guess.

That sounds reasonable to me.

-Andrew.