pgsql: amcheck: Support for different header sizes of short varlena dat

amcheck: Support for different header sizes of short varlena datum

In the heap, tuples may contain short varlena datum with both 1B header and 4B
headers. But the corresponding index tuple should always have such varlena's
with 1B headers. So, for fingerprinting, we need to convert.

Backpatch to all supported versions.

Discussion: https://postgr.es/m/flat/7bdbe559-d61a-4ae4-a6e1-48abdf3024cc%40postgrespro.ru
Author: Michael Zhilin
Reviewed-by: Alexander Lakhin, Andrey Borodin, Jian He, Alexander Korotkov
Backpatch-through: 12

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ab65dfb0fb2908989c0ef3330464b7af171e9728

Modified Files
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contrib/amcheck/expected/check_btree.out | 13 ++++++++++++
contrib/amcheck/sql/check_btree.sql | 11 ++++++++++
contrib/amcheck/verify_nbtree.c | 36 +++++++++++++++++++++++++++-----
3 files changed, 55 insertions(+), 5 deletions(-)