Instead of having this code duplicated in multiple places, let's have
a common interface for progress display.  If someday someone wishes to
display a cheezy progress bar instead then only one file will have to
be changed.

Note: I left merge-recursive.c out since it has a strange notion of
progress as it apparently increase the expected total number as it goes.
Someone with more intimate knowledge of what that is supposed to mean
might look at converting it to the common progress interface.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>