BUG #15689: Stemming of negation/not operator

The following bug has been logged on the website:

Bug reference:      15689
Logged by:          Ivan Viragine
Email address:      ivanviragine@gmail.com
PostgreSQL version: 10.3
Operating system:   MacOS and centOS
Description:

When using to_tsquery function it is stemming negation/not parts of the
query, where it probably shouldn't.
Some examples:

SELECT to_tsquery('english', 'car & !cars');
   to_tsquery   
----------------
 'car' & !'car'

SELECT to_tsquery('english', 'corporation & !corporative');
      to_tsquery      
----------------------
 'corpor' & !'corpor'

IMHO, it shouldn't stem cars to car, neither corporative to corpor. There
are several examples where it could lead to incorrect results.