First UPDATE under START TRANSACTION does nothing (nstate= nstate),
but anyway generates history. Since update vector is empty we get into
(!uvect->n_fields) branch which only adds history row, but does not do
update. After that we get current row with wrong (old) row_start value
and because of that second UPDATE tries to insert history row again
because it sees trx->id != row_start which is the guard to avoid
inserting multiple trx_id-based history rows under same transaction
(because we have same trx_id and we get duplicate error and this bug
demostrates that). But this try anyway fails because PK is based on
row_end which is constant under same transaction, so PK didn't change.

The fix moves vers_make_update() to an earlier stage of
calc_row_difference(). Therefore it prepares update vector before
(!uvect->n_fields) check and never gets into that branch, hence no
need to handle versioning inside that condition anymore.

Now trx->id and row_start are equal after first UPDATE and we don't
try to insert second history row.

== Cleanups and improvements ==

ha_innobase::update_row():

vers_set_fields and vers_ins_row are cleaned up into direct condition
check. SQLCOM_ALTER_TABLE check now is not used as this is dead code,
assertion is done instead.

upd_node->is_delete is set in calc_row_difference() just to keep
versioning code as much in one place as possible. vers_make_delete()
is still located in row_update_for_mysql() as this is required for
ha_innodbase::delete_row() as well.

row_ins_duplicate_error_in_clust():

Restrict DB_FOREIGN_DUPLICATE_KEY to the better conditions.
VERSIONED_DELETE is used specifically to help lower stack to
understand what caused current insert. Related to MDEV-29813.