[PATCH 0 of 2 PROPOSAL] Enable progress overview for conversion and other tasks

[timeless]

I'm running a convert (incremental) of VirtualBox's svn repo (using hg convert).
This is incredibly slow. I've enabled progress, but that as is just shows me mostly:
getting files [===========================================================>] 1/1 

sometimes:
scanning paths [==================>                                        ] 1/3 

occasionally:
getting files [=============================================>            ]  8/10 

rarely:
getting files [========>                                               ]  23/142 

And while there's technically a progress indicator for the general progress, 
because of the tight inner progress loop, you never see it. Oh, instead you get
a vertical progress meter in the wrong form (countdown with no reminder of
origin).

This series has two patches:
* Suppress the vertical countdown (one line summary and changesets to convert)
output from convert while progress is active
* Draft attempt to show the outer progress bar for at least a little while
when the inner meter is finished.

Note that the 20 time unit in the Draft doesn't actually result in 20s. You
get barely enough time to copy a progress bar:
converting [===>                                                   ]  2007/22205 
-- and that's only if you preselect in Terminal.app and rely on cmd-c.