Javascript fix.

Javascript fix.

Fix so that you can never have more than two checkboxes checked. (If this
happens, all but the current checkbox are unchecked.)

It used to be that one could view a PageHistory, check two boxes to view
a diff, then hit the back button.  (The originally checked two boxes are
still checked at this point.)  Checking a third box resulted in viewing
a diff between a quasi-random pair of versions selected from the three
which were selected.   Now clicking the third box results in the first
two being unchecked.

git-svn-id: svn://svn.code.sf.net/p/phpwiki/code/trunk@2679 96ab9672-09ca-45d6-a79d-3d69d39ca109