pgsql: Add new Tuplesortstate.removeabbrev function

Add new Tuplesortstate.removeabbrev function

This commit is the preparation to move abbreviation logic into
puttuple_common(). The new removeabbrev function turns datum1 representation
of SortTuple's from the abbreviated key to the first column value. Therefore,
it encapsulates the differential part of abbreviation handling code in
tuplesort_put*() functions, making these functions similar.

Discussion: https://postgr.es/m/CAPpHfdvjix0Ahx-H3Jp1M2R%2B_74P-zKnGGygx4OWr%3DbUQ8BNdw%40mail.gmail.com
Author: Alexander Korotkov
Reviewed-by: Pavel Borisov, Maxim Orlov, Matthias van de Meent
Reviewed-by: Andres Freund, John Naylor

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/cadfdd1edff103d696bbfcdd126c2dee516aed9b

Modified Files
--------------
src/backend/utils/sort/tuplesort.c | 137 ++++++++++++++++++++++++-------------
1 file changed, 89 insertions(+), 48 deletions(-)