use MergeRequest#closes_issues instead of rewriting code to retrieve closing issues on MergeRequest#issues_mentioned_but_not_closing

app/models/merge_request.rb

@@ -576,11 +576,7 @@ class MergeRequest < ActiveRecord::Base
     ext = Gitlab::ReferenceExtractor.new(project, current_user)
     ext.analyze(description)
 
-    issues = ext.issues
-    closing_issues = Gitlab::ClosingIssueExtractor.new(project, current_user).
-      closed_by_message(description)
-
-    issues - closing_issues
+    ext.issues - closes_issues
   end
 
   def target_project_path

spec/models/merge_request_spec.rb

@@ -283,12 +283,16 @@ describe MergeRequest, models: true do
   end
 
   describe '#issues_mentioned_but_not_closing' do
+    let(:closing_issue) { create :issue, project: subject.project }
+    let(:mentioned_issue) { create :issue, project: subject.project }
+
+    let(:commit) { double('commit', safe_message: "Fixes #{closing_issue.to_reference}") }
+
     it 'detects issues mentioned in description but not closed' do
-      mentioned_issue = create(:issue, project: subject.project)
-
       subject.project.team << [subject.author, :developer]
-      subject.description = "Is related to #{mentioned_issue.to_reference}"
+      subject.description = "Is related to #{mentioned_issue.to_reference} and #{closing_issue.to_reference}"
 
+      allow(subject).to receive(:commits).and_return([commit])
       allow(subject.project).to receive(:default_branch).
         and_return(subject.target_branch)