ui-atom: avoid DATE_STRFTIME

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

Since Atom accepts ISO8601 dates [1], we can use Git's
DATE_ISO8601_STRICT instead, which does get this right.  Additionally,
we never use the local timezone here so we can use the
date_mode_from_type() wrapper to simplify the code a bit.

[1] https://tools.ietf.org/html/rfc4287#section-3.3

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

cgit.h

@@ -29,11 +29,6 @@
 #undef isgraph
 #define isgraph(x) (isprint((x)) && !isspace((x)))
 
-/*
- * Dateformats used on misc. pages
- */
-#define FMT_ATOMDATE "%Y-%m-%dT%H:%M:%SZ"
-
 
 /*
  * Limits used for relative dates