Add translation for add comment btn title

app/helpers/notes_helper.rb

@@ -54,7 +54,7 @@ module NotesHelper
       class: 'add-diff-note js-add-diff-note-button',
       type: 'submit', name: 'button',
       data: diff_view_line_data(line_code, position, line_type),
-      title: 'Add a comment to this line' do
+      title: _('Add a comment to this line') do
       sprite_icon('comment', size: 12)
     end
   end

locale/gitlab.pot

@@ -1215,6 +1215,9 @@ msgstr ""
 msgid "Add a bullet list"
 msgstr ""
 
+msgid "Add a comment to this line"
+msgstr ""
+
 msgid "Add a general comment to this %{noteableDisplayName}."
 msgstr ""