Avoid creating duplicate ordered append paths

 Avoid creating duplicate ordered append paths
 
 In generate_orderedappend_paths(), the function does not handle the
 case where the paths in total_subpaths and fractional_subpaths are
 identical.  This situation is not uncommon, and as a result, it may
 generate two exactly identical ordered append paths.
 
 Fix by checking whether total_subpaths and fractional_subpaths contain
 the same paths, and skipping creation of the ordered append path for
 the fractional case when they are identical.
 
 Given the lack of field complaints about this, I'm a bit hesitant to
 back-patch, but let's clean it up in HEAD.
 
 Author: Richard Guo <guofenglinux@gmail.com>
 Reviewed-by: Andrei Lepikhov <lepihov@gmail.com>
 Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com>
 Discussion: https://postgr.es/m/CAMbWs4-OYsgA75tGGiBARt87G0y_z_GBTSLrzudcJxAzndYkYw@mail.gmail.com