Fix #5132: Modqueue displays active posts when excluding any search term

Fix a bug where searching for a negated tag inside the modqueue would show
active posts.

The bug was that in a search like this:

    Post.in_modqueue.user_tag_match("-solo")

The `in_modqueue` condition would get sucked inside the tag search and negated
when we tried to apply the negation operator to the "solo" tag. So effectively
the `in_modqueue` condition would get negated and we would end up searching for
everything not in the modqueue.

test/functional/modqueue_controller_test.rb

@@ -5,21 +5,25 @@ class ModqueueControllerTest < ActionDispatch::IntegrationTest
     setup do
       @admin = create(:admin_user)
       @user = create(:user)
-      @post = create(:post, is_pending: true)
     end
 
     context "index action" do
       should "render" do
+        create(:post, is_pending: true)
         get_auth modqueue_index_path, @admin
+
         assert_response :success
       end
 
       should "render for a json response" do
+        create(:post, is_pending: true)
         get_auth modqueue_index_path, @admin, as: :json
+
         assert_response :success
       end
 
       should "support the only= URL param" do
+        @post = create(:post, is_pending: true)
         get_auth modqueue_index_path(only: "rating"), @admin, as: :json
 
         assert_response :success
@@ -37,6 +41,23 @@ class ModqueueControllerTest < ActionDispatch::IntegrationTest
         assert_equal([post1.id, post2.id, post3.id], response.parsed_body.pluck("id"))
       end
 
+      should "filter negated tags correctly" do
+        post1 = create(:post, tag_string: "touhou", is_pending: false)
+        post2 = create(:post, tag_string: "touhou", is_pending: true)
+
+        get_auth modqueue_index_path(search: { tags: "-solo" }), @admin, as: :json
+        assert_response :success
+        assert_equal([post2.id], response.parsed_body.pluck("id"))
+
+        get_auth modqueue_index_path(search: { tags: "touhou -solo" }), @admin, as: :json
+        assert_response :success
+        assert_equal([post2.id], response.parsed_body.pluck("id"))
+
+        get_auth modqueue_index_path(search: { tags: "-touhou" }), @admin, as: :json
+        assert_response :success
+        assert_equal([], response.parsed_body.pluck("id"))
+      end
+
       should "include appealed posts in the modqueue" do
         @appeal = create(:post_appeal)
         get_auth modqueue_index_path, @admin