introduce "format" date-mode

This feeds the format directly to strftime. Besides being a
little more flexible, the main advantage is that your system
strftime may know more about your locale's preferred format
(e.g., how to spell the days of the week).

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

cache.h

@@ -1114,8 +1114,10 @@ struct date_mode {
 		DATE_ISO8601,
 		DATE_ISO8601_STRICT,
 		DATE_RFC2822,
+		DATE_STRFTIME,
 		DATE_RAW
 	} type;
+	const char *strftime_fmt;
 };
 
 /*