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>

strbuf.c

@@ -600,3 +600,22 @@ char *xstrdup_tolower(const char *string)
 	result[i] = '\0';
 	return result;
 }
+
+char *xstrvfmt(const char *fmt, va_list ap)
+{
+	struct strbuf buf = STRBUF_INIT;
+	strbuf_vaddf(&buf, fmt, ap);
+	return strbuf_detach(&buf, NULL);
+}
+
+char *xstrfmt(const char *fmt, ...)
+{
+	va_list ap;
+	char *ret;
+
+	va_start(ap, fmt);
+	ret = xstrvfmt(fmt, ap);
+	va_end(ap);
+
+	return ret;
+}