Introduce "scope" query_var to be used in WP_Query

[wonderboymusic]

After combing through a bunch of tickets, there seems to be a need for scoping queries to an initial WHERE condition.

Examples:
does the post have a password? or not? #20308
does the post have comments? or not? #25180

Rather than adding an endless number of new query vars for each case, maybe we should add one query_var: scope and add cases to it.

Scope is present in Active Record in Rails: ​http://guides.rubyonrails.org/active_record_querying.html#scopes, which is what made me think of this.

Something like:

$query = new WP_Query( array(
    'post_author' => 3,
    'scope' => 'has_comments'
) );

Perhaps even multiple:

$query = new WP_Query( array(
    'post_author' => 3,
    'scope' => 'has_comments,password_protected'
) );

or

$query = new WP_Query( array(
    'post_author' => 3,
    'scope' => array( 'has_comments', 'password_protected' )
) );

I will add an example patch later today.

[wonderboymusic]

implemented in 25193.diff​ - this would squash the need for #20308 or #25180. Added unit tests and fixed the horrendous whitespace on the post/query.php unit tests.

[wonderboymusic]

This did not receive the love I hoped

[wonderboymusic]

I opened this, I will reopen later if my future self decides it is worthwhile.