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>

cgit.h

@@ -32,8 +32,6 @@
 /*
  * Dateformats used on misc. pages
  */
-#define FMT_LONGDATE "%Y-%m-%d %H:%M:%S (%Z)"
-#define FMT_SHORTDATE "%Y-%m-%d"
 #define FMT_ATOMDATE "%Y-%m-%dT%H:%M:%SZ"