Add caching to get_page_by_title

[spacedmonkey]

Add caching to get_page_by_title so it doesn't call the posts table.

Related: #36711

Change seems OK, but get_page_by_title() has no automated test coverage at all. We'll need basic tests to introduce this kind of change.

Previously: #15805.

get_page_by_title() using WP_Query

Unit tests for 36905.2.patch

I propose to let WP_Query handle this.

I'd also propose to deprecate this function. Is not used by Core so plugins could use their own functions instead. Querying by title is not a very good idea so we would be discouraging developers to use this function (maybe just a little but more than nothing). @scribu said the same in here: https://core.trac.wordpress.org/ticket/20350#comment:1 I don't know if she would think the same today.

Anyway, here are my changes: Unit tests + changes inside the function (not deprecated though) so now it uses WP_Query instead.

Assigning to mark the good-first-bug as "claimed".

@igmoweb Can you please combine 36905.2.patch​ and 36905.2.tests.patch​ into a single patch?

Changes + Unit tests

Done.

Do we now upload changes + tests in the same file? I thought it was the other way.

Do we now upload changes + tests in the same file? I thought it was the other way.

We used to do it separately as they were in two different repo's, they're now combined so doing it in one patch is easier to keep track of everything (especially when things start getting into multiple iterations)

Some improvements + some more unit tests

patch 4 was mixed up with another ticket changes

36905.5.patch includes some improvements to the function:

• orderby is set to 'none' so it preserves the same order than the original function
• post_status is now an array with 'any' + all existent statuses even those that are excluded from search. This way, the query has less conditionals in WHERE clause and post_status field is not even filtered, like the original function.
• I added tests to check that the function query every single post status.