Add --date={local,relative,default}

Add --date={local,relative,default}

This adds --date={local,relative,default} option to log family of commands,
to allow displaying timestamps in user's local timezone, relative time, or
the default format.

Existing --relative-date option is a synonym of --date=relative; we could
probably deprecate it in the long run.

Signed-off-by: Junio C Hamano <junkio@cox.net>