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

[Michael Hudson]

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.

Cheers,
mwh