some diff (index in display_element, and bad page handling) needs to be incorporated into wporg

[hailin]

I discovered this inconsistency, partly due to merge, between wporg and wpcom, and believe they should be incorporated into wporg.

In function display_element in Walk class.

i should be set to –1 in after the recursive call returns.

Consider the following example:
E1

• e2
  
• - e3
  
• - - e4

And suppose children has order: e3, e2, e4 in the children array.

First we call display_element(E1), it will traverse children array, and find e2 as E1’s child; and the resulting array becomes e3, e4.

On the next recursive call, we are trying to find the child of e2 from the remaining children array.

Yet, with i--, it will start with e4, missing e3.

So this is a tricky case that is a little hard to imagine.
However, we already saw this happening with very complicated page hierarchy in wpcom.

Therefore, we have to reset index i to –1 so that on next call, it begins with index 0.

The catch and repair bad page diff is self-explanatory.

[hailin]

patch

[ryan]

(In [6552]) Page walk fixes from hailin. fixes #5581