Fix logic in link input widget

Previously pageExists thought matchingPages was an indexed object when
in fact it was an array. This didn't manifest as a bug as we subsequently
check the normalised URL in the correct manner (using indexOf), but it
did remove the optimisation of not running mw.Title if there was an exact match.

Change-Id: Ic616cbfa0d7ed5447e032dd1f772779db2dc19e5
Merge "Fix logic in link input widget"