show_date(): rename the "relative" parameter to "mode"

show_date(): rename the "relative" parameter to "mode"

Now, show_date() can print three different kinds of dates: normal,
relative and short (%Y-%m-%s) dates.

To achieve this, the "int relative" was changed to "enum date_mode
mode", which has three states: DATE_NORMAL, DATE_RELATIVE and
DATE_SHORT.

Since existing users of show_date() only call it with relative_date
being either 0 or 1, and DATE_NORMAL and DATE_RELATIVE having these
values, no behaviour is changed.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>