Bazaar dirstate locking

[Robert Collins]

They're separate nodes; bound branch commits do this:

local repository commit
push to remote repository
update remote branch pointer
update local [slave/subordinate] branch pointer.

This ordering allows efficient use of the local repository to do the
the data storage, and interrupted operations can be recovered from by
doing 'bzr update' (rather than it being committed locally but not
present remotely).

-Rob