Special case hiding refs/meta/config from Git clients

VisibleRefFilter requires a lot of server CPU to accurately provide
the correct listing to clients when they cannot read refs/*.

Since the default configuration is now to hide refs/meta/config,
use a special case in VisibleRefFilter that permits showing every
reference except refs/meta/config if a user can read every other
reference in the repository.

Change-Id: Iba12ee5ea1babcfeb71d378139ce2fcfb121b949