[MERGE] Add Branch.get_reference and BzrDir.get_branch_reference, plus other miscellany

[Andrew Bennetts]

John Arbash Meinel wrote:
> Andrew Bennetts wrote:
> > Sorry for the slightly unfocused bundle, but it's fairly short.
> > 
> > This is yet more stuff from the hpss branch.
> > 
> > -Andrew.
> > 
> > 
> 
> A little more discussion about what this was trying to do would have
> helped. One thing that might have saved me is realizing that
> 'get_reference' was part of BranchFormat, not Branch.

As mentioned on IRC, this work is mostly just Robert's (even though I happened
to be the one that extracted it and sent it off for review).  I think it would
be best for Robert to deal with these review comments, as I think he can explain
the purpose of the new APIs most clearly.

-Andrew.