pgsql: Teach pageinspect about nbtree deduplication.

Teach pageinspect about nbtree deduplication.

Add a new bt_metap() column to display the metapage's allequalimage
field. Also add three new columns to contrib/pageinspect's
bt_page_items() function:

* Add a boolean column ("dead") that displays the LP_DEAD bit value for
each non-pivot tuple.

* Add a TID column ("htid") that displays a single heap TID value for
each tuple. This is the TID that is returned by BTreeTupleGetHeapTID(),
so comparable values are shown for pivot tuples, plain non-pivot tuples,
and posting list tuples.

* Add a TID array column ("tids") that displays TIDs from each tuple's
posting list, if any. This works just like the "tids" column from
pageinspect's gin_leafpage_items() function.

No version bump for the pageinspect extension, since there hasn't been a
stable Postgres release since the last version bump (the last bump was
part of commit 58b4cb30).

Author: Peter Geoghegan
Discussion: https://postgr.es/m/CAH2-WzmSMmU2eNvY9+a4MNP+z02h6sa-uxZvN3un6jY02ZVBSw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/93ee38eade1b2b4964354b95b01b09e17d6f098d

Modified Files
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contrib/pageinspect/btreefuncs.c | 146 ++++++++++++++++++++++----
contrib/pageinspect/expected/btree.out | 7 ++
contrib/pageinspect/pageinspect--1.7--1.8.sql | 53 ++++++++++
doc/src/sgml/pageinspect.sgml | 120 +++++++++++++++------
4 files changed, 273 insertions(+), 53 deletions(-)