Post permalinks that include %category% should not query the category

[ryan]

3.1 now includes the category in the query when resolving a post permalink that includes %category%. 3.0 did not do this. This results in 'cat' and other category query vars being set for single post queries. This confuses some plugins and unnecessarily complicates the query. %category% is purely decorative since the post name is sufficient to find a post.

Discovered here:

​http://wordpress.org/support/topic/plugin-the-events-calendar-compatibility-with-wordpress-31?replies=11#post-1971053

3.0 query when resolving /%category%/%postname%/:

SELECT wp_30_posts.* FROM wp_30_posts WHERE 1=1 AND wp_30_posts.post_name = 'an-event' AND wp_30_posts.post_type = 'post' ORDER BY wp_30_posts.post_date DESC 

3.1 query:

SELECT wp_trunk_posts.* FROM wp_trunk_posts INNER JOIN wp_trunk_term_relationships ON (wp_trunk_posts.ID = wp_trunk_term_relationships.object_id) WHERE 1=1 AND wp_trunk_posts.post_name = 'an-event' AND ( wp_trunk_term_relationships.term_taxonomy_id IN (3) ) AND wp_trunk_posts.post_type = 'post' GROUP BY wp_trunk_posts.ID ORDER BY wp_trunk_posts.post_date DESC 

Note that is_category is being correctly set to false. Many plugins seem to be assuming a category query if 'cat' is set, however. One could argue that setting 'cat' is correct, but let's preserve the old expectation as well as get rid of the term query gymnastics for single post queries.

The SQL is indeed altered, but 'cat' isn't set, at least not in my testing.

check is_singular before altering query

No problems so far in testing. Single post queries with various permalinks look good. Category queries look good. Seems to fix The Events Calendar.

3.1 query with the patch:

SELECT wp_trunk_posts.* FROM wp_trunk_posts WHERE 1=1 AND wp_trunk_posts.post_name = 'an-event' AND wp_trunk_posts.post_type = 'post' ORDER BY wp_trunk_posts.post_date DESC 

Related: #16465

[17504] for trunk

[17505] for 3.1.