BUG: union_categorical fastpath sort
Follow up to a problem with #13846.   This path was tested, but the test just happened to be constructed in such a way it worked with the buggy implementation.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13899?src=pr) is 85.28% (diff: 100%)

> Merging [#13899](https://codecov.io/gh/pydata/pandas/pull/13899?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13899   diff @@
==========================================
  Files           139        139          
  Lines         50057      50057          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          42692      42692          
  Misses         7365       7365          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [45d54d0...b07a7d3](https://codecov.io/gh/pydata/pandas/compare/45d54d067ed9a0dfdb456f253fb0bede9969187c...b07a7d392f2f7a82846061db71ef34963dd81ecd?src=pr)
 ty sir!
