Track the number of presorted outer pathkeys in MergePath

Track the number of presorted outer pathkeys in MergePath

When creating an explicit Sort node for the outer path of a mergejoin,
we need to determine the number of presorted keys of the outer path to
decide whether explicit incremental sort can be applied.  Currently,
this is done by repeatedly calling pathkeys_count_contained_in.

This patch caches the number of presorted outer pathkeys in MergePath,
allowing us to save several calls to pathkeys_count_contained_in.  It
can be considered a complement to the changes in commit 828e94c9d.

Reported-by: David Rowley 
Author: Richard Guo 
Reviewed-by: Tender Wang 
Discussion: https://postgr.es/m/CAApHDvqvBireB_w6x8BN5txdvBEHxVgZBt=rUnpf5ww5P_E_ww@mail.gmail.com