Use `Commit#short_id` instead of `Commit.truncate_sha`

app/models/todo.rb

@@ -73,7 +73,7 @@ class Todo < ActiveRecord::Base
 
   def to_reference
     if for_commit?
-      Commit.truncate_sha(commit_id)
+      target.short_id
     else
       target.to_reference
     end

spec/models/todo_spec.rb

@@ -117,9 +117,11 @@ describe Todo, models: true do
 
   describe '#to_reference' do
     it 'returns the short commit id for commits' do
+      subject.project = project
       subject.target_type = 'Commit'
       subject.commit_id = commit.id
-      expect(subject.to_reference).to eq Commit.truncate_sha(commit.id)
+
+      expect(subject.to_reference).to eq commit.short_id
     end
 
     it 'returns reference for issuables' do