[MERGE] add a hook for Branch.set_last_revision_info()

[Andrew Bennetts]

James Henstridge wrote:
> One feature of the bzr-dbus plugin is to send a D-Bus signal over the
> session bus whenever the head revision of a branch changes.  It
> currently does this through the "set_rh" hook.

bb:comment

I haven't reviewed the code in detail yet, but this seems to have a lot of
overlap with https://lists.ubuntu.com/archives/bazaar/2008q2/040066.html

-Andrew.