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-shared.h

@@ -61,7 +61,7 @@ __attribute__((format (printf,1,2)))
 extern void cgit_print_error(const char *fmt, ...);
 __attribute__((format (printf,1,0)))
 extern void cgit_vprint_error(const char *fmt, va_list ap);
-extern const struct date_mode *cgit_date_mode(const char *format);
+extern const struct date_mode *cgit_date_mode(enum date_mode_type type);
 extern void cgit_print_age(time_t t, int tz, time_t max_relative);
 extern void cgit_print_http_headers(void);
 extern void cgit_redirect(const char *url, bool permanent);