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>

ui-commit.c

@@ -56,7 +56,7 @@ void cgit_print_commit(char *hex, const char *prefix)
 	cgit_close_filter(ctx.repo->email_filter);
 	html("</td><td class='right'>");
 	html_txt(show_date(info->author_date, info->author_tz,
-				cgit_date_mode(FMT_LONGDATE)));
+				cgit_date_mode(DATE_ISO8601)));
 	html("</td></tr>\n");
 	html("<tr><th>committer</th><td>");
 	cgit_open_filter(ctx.repo->email_filter, info->committer_email, "commit");
@@ -68,7 +68,7 @@ void cgit_print_commit(char *hex, const char *prefix)
 	cgit_close_filter(ctx.repo->email_filter);
 	html("</td><td class='right'>");
 	html_txt(show_date(info->committer_date, info->committer_tz,
-				cgit_date_mode(FMT_LONGDATE)));
+				cgit_date_mode(DATE_ISO8601)));
 	html("</td></tr>\n");
 	html("<tr><th>commit</th><td colspan='2' class='sha1'>");
 	tmp = oid_to_hex(&commit->object.oid);