[rfc] 'bzr missing' for checkout could use master branch for comparing histories

[Alexander Belchenko]

Because I heavily use checkouts I wonder why 'bzr missing' works by 
default with pull location only? I think there is sense to automatically 
choose branch to compare histories:

- if current branch is standalone or lightweight checkout then it will 
use stored pull location (as and before);
- if current branch is heavyweight checkout then it will use master 
branch location.

This change will allows to user easily inspect what difference between 
local and master branch. Especially when someone do local commits.

What you think about?

--
Alexander