backlink(.) should behave like backlink(<current page>)

backlink(.) should behave like backlink(<current page>)

Since commit c4d4cad3befbbd444d094cbeb0b6ebba3910a025, the single dot in
a pagespec can be used to mean the current page. While this worked
correctly in link() it didn't work in backlink(). Fix this by explicitly
checking the testpage in backlink against . and replacing it with the
current location if necessary.