Allow undoing edits that change content model if top

Allow undoing edits that change content model if top

This allows people to revert content model changes
using the undo button, provided that we are undoing
the topmost edit (Otherwise it may get confusing
if you try to undo an edit in the middle of the
history that changes content model).

Bug: T145044
Change-Id: Ic528f65d0dc581c4e241a22f19c512e02aeaa9e7