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>
diff --git a/ui-tag.c b/ui-tag.c
index b011198..6b838cb 100644
--- a/ui-tag.c
+++ b/ui-tag.c
@@ -77,7 +77,7 @@
 		if (info->tagger_date > 0) {
 			html("<tr><td>tag date</td><td>");
 			html_txt(show_date(info->tagger_date, info->tagger_tz,
-						cgit_date_mode(FMT_LONGDATE)));
+						cgit_date_mode(DATE_ISO8601)));
 			html("</td></tr>\n");
 		}
 		if (info->tagger) {