strbuf: add xstrfmt helper

strbuf: add xstrfmt helper

You can use a strbuf to build up a string from parts, and
then detach it. In the general case, you might use multiple
strbuf_add* functions to do the building. However, in many
cases, a single strbuf_addf is sufficient, and we end up
with:

  struct strbuf buf = STRBUF_INIT;
  ...
  strbuf_addf(&buf, fmt, some, args);
  str = strbuf_detach(&buf, NULL);

We can make this much more readable (and avoid introducing
an extra variable, which can clutter the code) by
introducing a convenience function:

  str = xstrfmt(fmt, some, args);

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