Inheritance operations must treat the OID column, if any, much like
regular user columns.  But MergeAttributesIntoExisting() neglected to
do that, leading to weird results after a table with OIDs is associated
to a parent with OIDs via ALTER TABLE ... INHERIT.

Report and patch by Amit Langote, reviewed by Ashutosh Bapat, some
adjustments by me.  It's been broken all along, so back-patch to
all supported branches.

Discussion: https://postgr.es/m/cb13cfe7-a48c-5720-c383-bb843ab28298@lab.ntt.co.jp