search: fix bug with negated user metatags.

Bug: searching for "filetype:jpg -user:evazion" would negate the
filetype:jpg metatag too. This was because the -user:<name> metatag was
negating the entire cumulative relation instead of just the user:<name>
clause.

test/unit/post_test.rb

@@ -2141,6 +2141,7 @@ class PostTest < ActiveSupport::TestCase
 
       assert_tag_match([posts[0]], "user:#{users[0].name}")
       assert_tag_match([posts[1]], "-user:#{users[0].name}")
+      assert_tag_match([posts[1]], "filetype:jpg -user:#{users[0].name}")
     end
 
     should "return posts for the approver:<name> metatag" do