Avoid entering The Loop when rendering Reader template

[westonruter]

Summary

This fixes an issue that was uncovered by @jackmcconnell in #1780 (comment).

If an overridden Reader mode template attempted to use The Loop to render the title or content, for example:

<?php if ( have_posts() ) : ?>
	<?php while ( have_posts() ) : the_post(); ?>
		<?php the_content(); ?>
	<?php endwhile; ?>
<?php endif; ?>

This would result in nothing being output because reader-template-loader.php was entering The Loop via the_post(). Since only one post is in The Loop in singular queries, the result was that have_posts() would return false and nothing would be output.

The fix is simply to directly populate the $post global and set up its global variables without advancing the current_post in the global WP_Query.

This fixes a regression that was introduced in #3781.

Checklist

• My pull request is addressing an open issue (please create one otherwise).
• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

[westonruter]

Build for testing: amp.zip (1.5.0-alpha-20200127T052304Z-584559dd7)

[pierlon]

No issue to report, LGTM.

[csossi]

CAn you add QA instructions, @pierlon ?

[westonruter]

This needs developer QA. As long as @pierlon confirmed the right behavior, this can move to Done.

[csossi]

thanks, @westonruter !