The attached patch (against 24) adds completing-read-function for
customising the behaviour of completing-read. See also the discussion in
emacs-devel: http://thread.gmane.org/gmane.emacs.devel/134000

(I am in the process of setting up an Emacs bzr repo and familiarising
 myself with bzr. I expect to be able to commit this myself in the
 beginning of February.)

Leo