add missing "format" function attributes

For most of our functions that take printf-like formats, we
use gcc's __attribute__((format)) to get compiler warnings
when the functions are misused. Let's give a few more
functions the same protection.

In most cases, the annotations do not uncover any actual
bugs; the only code change needed is that we passed a size_t
to transfer_debug, which expected an int. Since we expect
the passed-in value to be a relatively small buffer size
(and cast a similar value to int directly below), we can
just cast away the problem.

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

trace.c

@@ -75,6 +75,7 @@ static void trace_vprintf(const char *key, const char *fmt, va_list ap)
 	strbuf_release(&buf);
 }
 
+__attribute__((format (printf, 2, 3)))
 static void trace_printf_key(const char *key, const char *fmt, ...)
 {
 	va_list ap;