Make UserCache only look up a user once.

I noticed on special:listfiles/username, UserCache was doing queries
like select user_whatever from user where user_id in ( '1', '1', ...)
with the same user id 50 times (one for each result returned in the
special page). That seemed a little insane, so put the list of
users to query through a array_unique. (Quite likely the db would
optimize that query to not literally look up the same user 50 times,
but nonetheless it seems better to filter the list before then)

Change-Id: I80c8a359d0f7a53b2420ebcda641e594dd3c56e9
Merge "Make UserCache only look up a user once."