Regression in get_pages: hierarchical is disabled when using default arguments

[chrisjean]

With default arguments, get_pages has the following relevant arguments:

hierarchical: 1
parent: -1
child_of: 0

The 3.7 release included the following code:

if ( $parent && ! $child_of )
    $hierarchical = false;

This causes the default arguments listed above to change into:

hierarchical: false
parent: -1
child_of: 0

Thus, it is impossible to enable hierarchical if parent is set to -1, which it is by default.

[SergeyBiryukov]

Introduced in [25270].

[nacin]

Broken specifically in [25270], which was a follow-up to [25245]. See #25230.

#25230 really needed unit tests before [25245] landed. Let's make sure we have one here — that's very important.

[atimmer]

​25750.1.test.diff adds a test to test this behaviour.

[nacin]

In 25974:

Avoid resetting the 'hierarchical' argument in get_pages() when 'parent' is -1, the default.

Fixes a regression introduced in [25270]. Adds unit tests.

props chrisbliss18.
fixes #25750.

[nacin]

In 25975:

Move the unit test added in [25974] to the more appropriate file. see #25750.

[nacin]

In 25976:

Avoid resetting the 'hierarchical' argument in get_pages() when 'parent' is -1, the default.

Merges [25974] and [25975] to the 3.7 branch.

Fixes a regression introduced in [25270]. Adds unit tests.

props chrisbliss18.
fixes #25750.