progress: move cached debug flag from progress.progbar to scmutil.progress

Authored by martinvonz.

Description

progress: move cached debug flag from progress.progbar to scmutil.progress

It's simpler this way. One possible drawback (and a possisble
advantage) is that we now check the debug flag once per topic, so
processes that generate new topics all the time will still check the
flag frequently.

Differential Revision: https://phab.mercurial-scm.org/D5528