Improve output for extra queries in specs

Previously, this used `Array#-`, which would remove all queries that matches the
query text in the original set.

However, sometimes we have a problem with parameterised queries, where the query
text is identical both times, so we'd run a query N times instead of once, and
it would be hidden from the output.

Replace the logic to only remove a given query N times from the actual log,
where N is the number of times it appears in the expected log.

spec/requests/api/merge_requests_spec.rb

@@ -172,15 +172,15 @@ describe API::MergeRequests do
 
     context "when authenticated" do
       it 'avoids N+1 queries' do
-        control_count = ActiveRecord::QueryRecorder.new do
+        control = ActiveRecord::QueryRecorder.new do
           get api("/projects/#{project.id}/merge_requests", user)
-        end.count
+        end
 
         create(:merge_request, state: 'closed', milestone: milestone1, author: user, assignee: user, source_project: project, target_project: project, title: "Test", created_at: base_time)
 
         expect do
           get api("/projects/#{project.id}/merge_requests", user)
-        end.not_to exceed_query_limit(control_count)
+        end.not_to exceed_query_limit(control)
       end
 
       it "returns an array of all merge_requests" do

spec/support/query_recorder.rb

@@ -69,10 +69,17 @@ RSpec::Matchers.define :exceed_query_limit do |expected|
     @recorder.count
   end
 
+  def count_queries(queries)
+    queries.each_with_object(Hash.new(0)) { |query, counts| counts[query] += 1 }
+  end
+
   def log_message
     if expected.is_a?(ActiveRecord::QueryRecorder)
-      extra_queries = (expected.log - @recorder.log).join("\n\n")
-      "Extra queries: \n\n #{extra_queries}"
+      counts = count_queries(expected.log)
+      extra_queries = @recorder.log.reject { |query| counts[query] -= 1 unless counts[query].zero? }
+      extra_queries_display = count_queries(extra_queries).map { |query, count| "[#{count}] #{query}" }
+
+      (['Extra queries:'] + extra_queries_display).join("\n\n")
     else
       @recorder.log_message
     end