Don't lose column values on REPACK

Don't lose column values on REPACK

Commit 28d534e2ae0a introduced reform_tuple() with a fast path that
returns the source tuple verbatim when no dropped columns require fixing
up.  I (Álvaro) failed to realize that this broke handling of columns
with a 'missingval' defined: after a VACUUM FULL, CLUSTER, or REPACK
operation, the catalogued missingval is thrown away, so the tuples are
no longer correct.

Fix by forcing the rewrite when the tuple is shorter than the tuple
descriptor.

Author: Satya Narlapuram <satyanarlapuram@gmail.com>
Discussion: https://postgr.es/m/CAHg+QDeoccU5CudrJpmSKZfKZ1gRMNY=5BxSC=JpHgkonzgcOw@mail.gmail.com