pgsql: Refactor nbtree high key truncation.

Refactor nbtree high key truncation.

Simplify _bt_truncate(), the routine that generates truncated leaf page
high keys. Remove a micro-optimization that avoided a second palloc0()
call (this was used when a heap TID was needed in the final pivot tuple,
though only when the index happened to not be an INCLUDE index).

Removing this dubious micro-optimization allows _bt_truncate() to use
the index_truncate_tuple() indextuple.c utility routine in all cases.
This was already the common case.

This commit is a HEAD-only follow up to bugfix commit 4b42a899.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7c2dbc691c3e19b7d33c78f6c8aacc6e0cab510b

Modified Files
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src/backend/access/nbtree/nbtutils.c | 130 ++++++++++++-----------------------
1 file changed, 45 insertions(+), 85 deletions(-)