Fix costing bug in MergeAppend

When building a MergeAppendPath which has child paths that are not
sorted correctly for the MergeAppend's sort order, we apply the cost of
sorting those paths to the MergeAppendPath costs.

Here we fix a bug where the number of tuples specified that needed to be
sorted was effectively pg_class.reltuples rather than the number of
expected row in the subpath.  This effectively penalizes MergeAppend
plans any time any filter is present on the MergeAppend subpath as the
sort cost added is to sort all tuples in the table rather than just the
ones expected the path to return.

This did not affect UNION ALL type queries as the RelOptInfo tuples is
set from the subquery's path rows.  It does affect MergeAppends uses for
inheritance and partitioned tables.

This is a long-standing bug introduced when MergeAppend was first added
in 11cad29c9.  No backpatch as this could result in plan changes.

Author: Alexander Kuzmenkov
Reviewed-by: Ashutosh Bapat, Aleksander Alekseev, David Rowley
Discussion: https://postgr.es/m/CALzhyqyhoXQDR-Usd_0HeWk%3DuqNLzoVeT8KhRoo%3DpV_KzgO3QQ%40mail.gmail.com

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