nicer display of thin pack completion

nicer display of thin pack completion

In the same spirit of prettifying Git's output display for mere mortals,
here's a simple extension to the progress API allowing for a final
message to be provided when terminating a progress line, and use it for
the display of the number of objects needed to complete a thin pack,
saving yet one more line of screen display.

Signed-off-by: Nicolas Pitre <redacted>
Signed-off-by: Junio C Hamano <redacted>