strbuf_humanise: use unsigned variables

strbuf_humanise: use unsigned variables

All of the numeric formatting done by this function uses
"%u", but we pass in a signed "int". The actual range
doesn't matter here, since the conditional makes sure we're
always showing reasonably small numbers. And even gcc's
format-checker does not seem to mind. But it's potentially
confusing to a reader of the code to see the mismatch.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>