Comment 0 for bug 364908

Hooks and other activity running in the server should be able to provide
progress indicators.

Key uses are for things like reconcile progress, and pre_tip change
hooks that run test suites and the like.

Sketchily we should make sure anything that may invoke complex hooks has
a streaming body; and get trace.note() on the server, and
ui_factory.get_nested_progressbar() to work with that body.

 affects bzr
 importance high