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
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2