Fix diffs in text part of email-on-push messages

`safe_diff_files` directly modifies @message.diffs and was being called inside the HTML template
of the e-mail. By the time the text e-mail rendering occurs, the objects were already
wrapped by GitLab's diff helpers. This MR makes it explicit that the diff wrapper
will be used in both the text and HTML e-mail templates.

Closes gitlab-org/gitlab-ce#14497

app/views/notify/repository_push_email.html.haml

@@ -46,7 +46,7 @@
             = diff.new_path
 
   - unless @message.disable_diffs?
-    - diff_files = safe_diff_files(@message.diffs, @message.diff_refs)
+    - diff_files = @message.diffs
 
     %h4 Changes:
     - diff_files.each_with_index do |diff_file, i|

app/views/notify/repository_push_email.text.haml

@@ -28,17 +28,17 @@
     \
     \
     Changes:
-    - @message.diffs.each do |diff|
+    - @message.diffs.each do |diff_file|
       \
       \=====================================
-      - if diff.deleted_file
-        #{diff.old_path} deleted
-      - elsif diff.renamed_file
-        #{diff.old_path} → #{diff.new_path}
+      - if diff_file.deleted_file
+        #{diff_file.old_path} deleted
+      - elsif diff_file.renamed_file
+        #{diff_file.old_path} → #{diff_file.new_path}
       - else
-        = diff.new_path
+        = diff_file.new_path
       \=====================================
-      != diff.diff
+      != diff_file.diff.diff
   - if @message.compare_timeout
     \
     \

lib/gitlab/email/message/repository_push.rb

@@ -6,6 +6,7 @@ module Gitlab
         attr_reader :author_id, :ref, :action
 
         include Gitlab::Routing.url_helpers
+        include DiffHelper
 
         delegate :namespace, :name_with_namespace, to: :project, prefix: :project
         delegate :name, to: :author, prefix: :author
@@ -38,7 +39,7 @@ module Gitlab
         end
 
         def diffs
-          @diffs ||= (compare.diffs if compare)
+          @diffs ||= (safe_diff_files(compare.diffs, diff_refs) if compare)
         end
 
         def diffs_count

spec/lib/gitlab/email/message/repository_push_spec.rb

@@ -57,7 +57,7 @@ describe Gitlab::Email::Message::RepositoryPush do
 
     describe '#diffs' do
       subject { message.diffs }
-      it { is_expected.to all(be_an_instance_of Gitlab::Git::Diff) }
+      it { is_expected.to all(be_an_instance_of Gitlab::Diff::File) }
     end
 
     describe '#diffs_count' do