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>