[RFC] patch series to sketch a less verbose and frightening output

  Follow 4 patches that rework progress.c a bit so that one stage only
takes one line of output with a progress bar.

  Some of the patch fix (IMHO) pointless lines to print on screen (even
for debugging purposes).

  With those patches, running git gc looks like:

    $ git gc
    Counting objects    : 10095
    Deltifying objects  : [===================================================] 100%
    Writing objects     : [==========================>                        ] 50%

    [...]

    Counting objects    : 10095
    Deltifying objects  : [===================================================] 100%
    Writing objects     : [===================================================] 100%
    Pruning objects     : [===================================================] 100%

I don't believe those patches are 100% okay yet, because it doesn't plays that nice during a clone (it's not horribly broken, merely surprising). But it sketches what it could look like.