[merge][rfc] branch --stacked should set up to use a stackable format

[Andrew Bennetts]

Andrew Bennetts has voted comment.
Status is now: Semi-approved
Comment:
The code itself seems ok.  The increasingly large number of combinations 
of format features (rich root * external lookups * stacked...) is a 
little bit of a worry.  Ideally we'd handle the combinations more 
elegantly than lots of "if" statements in the code, and ideally we'd 
have tests for all combinations.

I can't think of any easy alternatives, though.

If I can stop worrying about that, this will be bb:approve :)

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3Ce01316480807240729g1dfdab0bn565ae9de34faf60d%40mail.gmail.com%3E