Fix bug with cross-reference note on commit

It should not set noteable_id if noteable_type is Commit
We have Note#commit_id for this
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/models/note.rb

@@ -72,14 +72,20 @@ class Note < ActiveRecord::Base
     # +noteable+ was referenced from +mentioner+, by including GFM in either +mentioner+'s description or an associated Note.
     # Create a system Note associated with +noteable+ with a GFM back-reference to +mentioner+.
     def create_cross_reference_note(noteable, mentioner, author, project)
-      create({
-        noteable: noteable,
-        commit_id: (noteable.sha if noteable.respond_to? :sha),
+      note_options = {
         project: project,
         author: author,
         note: "_mentioned in #{mentioner.gfm_reference}_",
         system: true
-      }, without_protection: true)
+      }
+
+      if noteable.kind_of?(Commit)
+        note_options.merge!(noteable_type: 'Commit', commit_id: noteable.id)
+      else
+        note_options.merge!(noteable: noteable)
+      end
+
+      create(note_options, without_protection: true)
     end
 
     def create_assignee_change_note(noteable, project, author, assignee)

spec/models/note_spec.rb

@@ -250,6 +250,16 @@ describe Note do
       its(:project) { should == project }
       its(:note) { should == "_mentioned in merge request !#{mergereq.iid}_" }
     end
+
+    context 'commit from issue' do
+      subject { Note.create_cross_reference_note(commit, issue, author, project) }
+
+      it { should be_valid }
+      its(:noteable_type) { should == "Commit" }
+      its(:noteable_id) { should be_nil }
+      its(:commit_id) { should == commit.id }
+      its(:note) { should == "_mentioned in issue ##{issue.iid}_" }
+    end
   end
 
   describe '#cross_reference_exists?' do