(bug 49840) Moving a page with subpages may lead in the page being moved twice

This happens if a page is moved to a subpage of itself and the option
"move subpages" is enabled, since the freshly moved page will be found
as being one of its subpages and moved a second time as such.

Bug: 49840
Change-Id: I318aab8c3c6e7ae718a3366ab00aba6e5420529b