BUG #11803: avoid "distinct" logic if "limit 1" specified

The following bug has been logged on the website:

Bug reference:      11803
Logged by:          Korenberg Mark
Email address:      socketpair@gmail.com
PostgreSQL version: 9.3.1
Operating system:   Linux
Description:

In other words,

query:
select * from tablename order by id limit 1

and query:
select distinct * from tablename order by id limit 1

should be planned exactly (as if distinct was not specified)
-----------------------
https://github.com/ponyorm/pony/issues/90
http://doc.ponyorm.com/queries.html#comment-1654734639