edit: Detect concurrent updates during interactive nation edit

edit: Detect concurrent updates during interactive nation edit

Unlike other edits, editing a country modifies the object in-place.
It has to, because editing may send bulletins, which updates nat_tgms,
and writing back a copy would clobber these updates, triggering a
seqno mismatch oops.

However, the in-place editing neglects to detect concurrent updates.
Fix that.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>