Problem:
Trying to match the search highlight groups to the Normal highlight for
the window can fail when the message highlighting contains a fg/bg that
the Normal highlight doesn't (like an error message in cmd will have
ErrorMsg highlight instead of MsgArea - which is Normal in cmd.)
Solution:
Link the search highlight groups to an empty group in 'winhighlight'
thus disabling them instead of overriding them with Normal/MsgArea/etc.
ea70d2ad85