pgsql: Stabilize repack_decode isolation test.

Stabilize repack_decode isolation test.

Commit 7164dcb66532 added the repack_decode isolation test, whose
s1_decode step counts every change decoded from the slot. A concurrent
autovacuum transaction that modifies only system catalogs is decoded
as an empty BEGIN/COMMIT pair, so the count could come out larger
than expected.

Fix by passing skip-empty-xacts to test_decoding. We pass it, along
with include-xids=0, to s1_decode_updates as well so that both steps
use the same options for consistency, though that step shows only
UPDATE lines and wasn't affected.

Per buildfarm animal skink.

Reported-by: Alexander Lakhin <exclusion(at)gmail(dot)com>
Discussion: https://postgr.es/m/d704cde4-b1f2-48dc-8c5e-659d1128d286@gmail.com
Backpatch-through: 19

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/adffcab923e1cd3d2a0fd0b21eb4a91f080065a3

Modified Files
--------------
src/test/modules/injection_points/expected/repack_decode.out | 8 +++++---
src/test/modules/injection_points/specs/repack_decode.spec | 6 ++++--
2 files changed, 9 insertions(+), 5 deletions(-)