Fix REPACK CONCURRENTLY for stored generated columns

Fix REPACK CONCURRENTLY for stored generated columns

In order to replay concurrent changes, REPACK CONCURRENTLY needs the
pg_attrdef tuples for the transient table to be there, in case a tuple
is modified concurrently with REPACK and requires to store the value
from the generated column (which, with the current arrangements, means
all tuples concurrently updated or inserted).  Fix by creating a copy of
them from the original table.  Add a test that tickles the bug.

Author: Antonin Houska <ah@cybertec.at>
Reported-by: Ewan Young <kdbase.hack@gmail.com>
Diagnosed-by: Ewan Young <kdbase.hack@gmail.com>
Reviewed-by: Ewan Young <kdbase.hack@gmail.com>
Backpatch-through: 19
Discussion: https://postgr.es/m/CAON2xHMrELwx9vKg6niSf8fMBA=-MGXmG=MPQU6+vMVhGjF8kQ@mail.gmail.com