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>

revision.h

@@ -63,8 +63,8 @@ struct rev_info {
 
 	/* Format info */
 	unsigned int	shown_one:1,
-			abbrev_commit:1,
-			relative_date:1;
+			abbrev_commit:1;
+	enum date_mode date_mode;
 
 	const char **ignore_packed; /* pretend objects in these are unpacked */
 	int num_ignore_packed;