Ignore subfolders in the _posts folder.

This reverts only the changes made in https://github.com/jekyll/jekyll/pull/2633

It was performed by running `git revert -m 1 18c033dc5c00b2ce0b3528cb310bf3988377e39f` and manually solving the merge conflict to preserve later changes. 
Can we keep this test and ensure that the categories are *not* added?