pgsql: Clear out values from dropped columns during REPACK (CONCURRENTL

Clear out values from dropped columns during REPACK (CONCURRENTLY)

When repacking a table concurrently, some border conditions can cause
a concurrent update to propagate the original old tuple to the transient
table (such as a trigger that returns OLD). That's essentially just
wasted space, which REPACK is there to remove. Prevent that by setting
the isnull flag to true for any column cataloged as dropped in tuples
being "replayed" as concurrent updates.

Reported-by: Radim Marek <radim(at)boringsql(dot)com>
Reviewed-by: Antonin Houska <ah(at)cybertec(dot)at>
Backpatch: 19
Discussion: https://postgr.es/m/CAJgoLkK2UBzB1J9buCsSUbjf7bOqz-o_0=CeiTruU789BhBw-Q@mail.gmail.com

Branch
------
REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/10c67bcb2da6acb16ce77ff1a498532b26538dcc

Modified Files
--------------
src/backend/commands/repack.c | 36 +++++++----
.../injection_points/expected/repack_dropped.out | 47 ++++++++++++++
src/test/modules/injection_points/meson.build | 1 +
.../injection_points/specs/repack_dropped.spec | 71 ++++++++++++++++++++++
4 files changed, 144 insertions(+), 11 deletions(-)