Fix tuple_fraction calculation in generate_orderedappend_paths()

Fix tuple_fraction calculation in generate_orderedappend_paths()

6b94e7a6da adjusted generate_orderedappend_paths() to consider fractional
paths.  However, it didn't manage to interpret the tuple_fraction value
correctly.  According to the header comment of grouping_planner(), the
tuple_fraction >= 1 specifies the absolute number of expected tuples.  That
number must be divided by the expected total number of tuples to get the
actual fraction.

Even though this is a bug fix, we don't backpatch it.  The risks of the side
effects of plan changes on stable branches are too high.

Reported-by: Andrei Lepikhov <lepihov@gmail.com>
Discussion: https://postgr.es/m/3ca271fa-ca5c-458c-8934-eb148622b270%40gmail.com
Author: Andrei Lepikhov <lepihov@gmail.com>
Reviewed-by: Junwang Zhao <zhjwpku@gmail.com>
Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>