strbuf: create strbuf_humanise_bytes() to show byte sizes

strbuf: create strbuf_humanise_bytes() to show byte sizes

Humanization of downloaded size is done in the same function as text
formatting in 'process.c'. The code cannot be reused easily elsewhere.

Separate text formatting from size simplification and make the
function public in strbuf so that it can easily be used by other
callers.

We now can use strbuf_humanise_bytes() for both downloaded size and
download speed calculation. One of the drawbacks is that speed will
now look like this when download is stalled: "0 bytes/s" instead of
"0 KiB/s".

Signed-off-by: Antoine Pelisse <apelisse@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>