[MERGE] Add user doc for stacked branches

[Martin Pool]

Martin Pool has voted approve.
Status is now: Approved
Comment:
It's good documentation.  It does point out a few points in how stacking 
works at present.

As I posted separately, at present 'branch --stacked' will fail coming 
from a branch not already in the development1 format because it's trying 
to create one of the same format.

As Aaron said, making branching from a stacked branch stack against the 
same reference is probably dangerous behaviour so should be changed.  So 
then you will only get a stacked branch if you specifically request it 
on the command line or a policy setting.

But then consider this case:

   bzr init-repo ~/project
   bzr branch --stacked http://example.com/project project/trunk
   bzr branch project/trunk project/mywork

It seems like this would cause mywork to be a non-stacked branch and 
therefore to pull down all of the history.  In fact there is code in 
_show_push_branch that seems to cause this to happen.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C4876C902.7040004%40canonical.com%3E