User specfied sort added, along with bug.cmp_last_modified.

Hmm, perhaps you don't want the last comment date, but e.g. the last
time one of the bug attributes are changed.  In that case, I suggest
  bzr log .be/bugs/9ce2f015-8ea0-43a5-a03d-fc36f6d202fe/

Maybe log(file) functionality should be incorperated into libbe/rcs...
Perhaps accessed through a --history.  I'm not sure I remember enough
Arch to do that ;).