use xstrfmt to replace xmalloc + sprintf

use xstrfmt to replace xmalloc + sprintf

This is one line shorter, and makes sure the length in the
malloc and sprintf steps match.

These conversions are very straightforward; we can drop the
malloc entirely, and replace the sprintf with xstrfmt.

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