progress: split up _print() method in bar-updating and debug-printing
I just thought this was clearer, but it turned out to also simplify
the next patch.
This seems to have sped up `hg perfprogress` from 1.85 s to 1.78 s.
Differential Revision: https://phab.mercurial-scm.org/D5529
#require docutils gettext
$ $TESTDIR/check-gendoc zh_TW
checking for parse errors