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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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/50f8611d073e255c9d5b3792965595879c22c610

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(-)