blame: don't overflow time buffer

When showing the raw timestamp, we format the numeric
seconds-since-epoch into a buffer, followed by the timezone
string. This string has come straight from the commit
object. A well-formed object should have a timezone string
of only a few bytes, but we could be operating on data
pushed by a malicious user.

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

builtin/blame.c

@@ -1598,7 +1598,7 @@ static const char *format_time(unsigned long time, const char *tz_str,
 	int tz;
 
 	if (show_raw_time) {
-		sprintf(time_buf, "%lu %s", time, tz_str);
+		snprintf(time_buf, sizeof(time_buf), "%lu %s", time, tz_str);
 	}
 	else {
 		tz = atoi(tz_str);