Add diff hunks to notification emails on MR

Added diff hunks to notification emails of messages on merge requests. This
provides code context to the note. Uses existing template for formatting
a diff for email (from repository push notifications).

CHANGELOG.md

@@ -797,6 +797,7 @@ entry.
 ## 8.11.0 (2016-08-22)
 
   - Use test coverage value from the latest successful pipeline in badge. !5862
+  - Add git diff context to notifications of new notes on merge requests !5855 (hoopes)
   - Add test coverage report badge. !5708
   - Remove the http_parser.rb dependency by removing the tinder gem. !5758 (tbalthazar)
   - Add Koding (online IDE) integration

app/views/notify/note_merge_request_email.html.haml

+= content_for :head do
+  = stylesheet_link_tag 'mailers/highlighted_diff_email'
+
 - if @note.diff_note? && @note.diff_file
   %p.details
     New comment on diff for
     = link_to @note.diff_file.file_path, @target_url
     \:
 
+    .diff-content.code.js-syntax-highlight
+      %table
+        - Discussion.new([@note]).truncated_diff_lines.each do |line|
+          = render "projects/diffs/line", line: line, diff_file: @note.diff_file, plain: true
+
 = render 'note_message'

app/views/notify/repository_push_email.html.haml

 = content_for :head do
-  = stylesheet_link_tag 'mailers/repository_push_email'
+  = stylesheet_link_tag 'mailers/highlighted_diff_email'
 
 %h3
   #{@message.author_name} #{@message.action_name} #{@message.ref_type} #{@message.ref_name}

spec/mailers/notify_spec.rb

@@ -580,8 +580,10 @@ describe Notify do
       let(:note_author) { create(:user, name: 'author_name') }
       let(:note) { create(:note, project: project, author: note_author) }
 
-      before :each do
-        allow(Note).to receive(:find).with(note.id).and_return(note)
+      before do |example|
+        unless example.metadata[:skip_before]
+          allow(Note).to receive(:find).with(note.id).and_return(note)
+        end
       end
 
       shared_examples 'a note email' do
@@ -663,6 +665,19 @@ describe Notify do
         end
       end
 
+      describe "on a merge request with diffs", :skip_before do
+        let(:merge_request) { create(:merge_request_with_diffs) }
+        let(:note_with_diff) {create(:diff_note_on_merge_request)}
+
+        before(:each) { allow(note_with_diff).to receive(:noteable).and_return(merge_request) }
+        subject { Notify.note_merge_request_email(recipient.id, note_with_diff.id) }
+
+        it 'includes diffs with character-level highlighting' do
+          expected_line_text = Discussion.new([note_with_diff]).truncated_diff_lines.first.text
+          is_expected.to have_body_text expected_line_text
+        end
+      end
+
       describe 'on an issue' do
         let(:issue) { create(:issue, project: project) }
         let(:note_on_issue_path) { namespace_project_issue_path(project.namespace, project, issue, anchor: "note_#{note.id}") }