advice: pass varargs to strbuf_vaddf, not strbuf_addf

advice: pass varargs to strbuf_vaddf, not strbuf_addf

The advise() function takes a variable number of arguments
and converts them into a va_list object to pass to strbuf
for handling. However, we accidentally called strbuf_addf
(that takes a variable number of arguments) instead of
strbuf_vaddf (that takes a va_list).

This bug dates back to v1.7.8.1-1-g23cb5bf, but we never
noticed because none of the current callers passes a string
with a format specifier in it. And the compiler did not
notice because the format string is not available at
compile time.

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