Avoid DATE_STRFTIME for long/short dates

Git's DATE_STRFTIME ignores the timezone argument and just uses the
local timezone regardless of whether the "local" flag is set.

Since our existing FMT_LONGDATE and FMT_SHORTDATE are pretty-much
perfect matches to DATE_ISO8601 and DATE_SHORT, switch to taking a
date_mode_type directly in cgit_date_mode().

Signed-off-by: John Keeping <john@keeping.me.uk>