introduce "format" date-mode

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>