progress: assemble percentage and counters in a strbuf before printing

progress: assemble percentage and counters in a strbuf before printing

The following patches in this series want to handle the progress bar's
title and changing parts (i.e. the counter and the optional percentage
and throughput combined) differently, and need to know the length
of the changing parts of the previously displayed progress bar.

To prepare for those changes assemble the changing parts in a separate
strbuf kept in 'struct progress' before printing.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>