Support 'raw' date format

Support 'raw' date format

Talking about --date, one thing I wanted for the 1234567890 date was to
get things in the raw format. Sure, you get them with --pretty=raw, but it
felt a bit sad that you couldn't just ask for the date in raw format.

So here's a throw-away patch (meaning: I won't be re-sending it, because I
really don't think it's a big deal) to add "--date=raw". It just prints
out the internal raw git format - seconds since epoch plus timezone (put
another way: 'date +"%s %z"' format)

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>