[FileItemRolesUpdater] Fix calculation of requested indexes

In case the last and/or first page (a page being n=m_maximumVisibleItems
items) overlapped the already inserted items, one item for each range
would be inserted a second time, e.g. for the first page and last page:

`beginExtendedVisibleRange == 0` => `endFirstPage := 0` => `0 <= 0` is
inserted again.

`count == 100; endExtendedVisibleRange == 99; m_maximumVisibleItems == 20`
=> `beginLastPage := 99` => `99 < 100` is inserted again.