wt-status: use "format" function attribute for status_printf

These functions could benefit from the added compile-time
safety of having the compiler check printf arguments.

Unfortunately, we also sometimes pass an empty format string,
which will cause false positives with -Wformat-zero-length.
In this case, that warning is wrong because our function is
not a no-op with an empty format: it may be printing
colorized output along with a trailing newline.

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