wp_update_plugins irregular transient handling.

[Cybr]

When creating $updates in wp_update_plugins(), ​its properties are populated immediately.

However, one property is missing, which is used later: checked.

When hooking into pre_set_site_transient_update_plugins, the checked property will be available sporadically because of this, depending on whether the $time_not_changed && ! $extra_stats ​branch is executed or not.

Another noteworthy issue with this function is that its implementation of set_site_transient( 'update_plugins', ... ); uses both the values $current and $updates. They store ​different objects at different times (​1, ​2).

Prepopulating both values with the same object properties would help prevent object-typing issues.

[siliconforks]

Related: #44118

### Ticket: https://core.trac.wordpress.org/ticket/61055

## Description
Ensure consistent handling of 'checked' property in wp_update_plugins() function.

• This PR addresses the irregular handling of the transient in the wp_update_plugins() function, ensuring that the 'checked' property is always available when hooking into pre_set_site_transient_update_plugins.
• It also ensures consistency between the $current and $updates objects, preventing potential issues caused by different object properties at different times.

## Changes Made

• Initialize the 'checked' property in the $current object if it is not already set.
• Ensure that the 'checked' property is always assigned to the $updates object.
• Prepopulate both $current and $updates with the same 'checked' property to prevent object-typing issues.
• Update the set_site_transient('update_plugins', $updates) to store the $updates object consistently.

Note that there is already an existing patch (which is intended to address this problem) in this ticket:

https://core.trac.wordpress.org/ticket/44118

[Cybr]

This might be a duplicate of #44118; though addressing a different issue, its resolution appears to be the same.

[siliconforks]

Replying to Cybr:

This might be a duplicate of #44118; though addressing a different issue, its resolution appears to be the same.

#44118 basically has the same root cause - the checked property is missing from the transient, which causes additional (unnecessary) update checks to be performed.

[nhrrob]

We are very close to RC1.
Looks like it still needs testing.
Punting to 6.7