use xstrfmt in favor of manual size calculations

use xstrfmt in favor of manual size calculations

In many parts of the code, we do an ugly and error-prone
malloc like:

  const char *fmt = "something %s";
  buf = xmalloc(strlen(foo) + 10 + 1);
  sprintf(buf, fmt, foo);

This makes the code brittle, and if we ever get the
allocation wrong, is a potential heap overflow. Let's
instead favor xstrfmt, which handles the allocation
automatically, and makes the code shorter and more readable.

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