[CHG] base_import: sort-of reduce o2m depth by 1 level
[CHG] base_import: sort-of reduce o2m depth by 1 level

Before, once reaching level 0 all of an O2M's fields would still be
selected, so we would really be selecting depth+1 levels of fields,
which makes for an extremely busy list as it's a combinatorial
explosion.

Change this so at level 0 the only field selected from a field is an
xid. Not sure that's even necessary though, do xids really make sense
for o2ms?

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo