Prevent Page.change_category from readding newCat.

Currently change_category also adds newCat if it
is already present at the target page. This
behavior had to be fixed manually by calling
functions like category.py and causes extra code
and complexity.

Change-Id: I95ba291e78c2f187f4d4a881b39fa44096cca9b6
Merge "Prevent Page.change_category from readding newCat."