Extend FileRepo::findFile private option to accept a User object

Callers may want to test visibility for a user other than $wgUser when
specifying the 'private' option. It seems a natural extension to allow
'private' to be a User object, with boolean true retaining its old
meaning of $wgUser.

Change-Id: Idbed0f3055c0135b5c11068de1bf1ef668e13456