Merge branch 'add-mirror-helper' into 'master'

Add mirror_helper

Add missing file from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/138.

See merge request !150

app/helpers/mirror_helper.rb

+module MirrorHelper
+  def branch_diverged_tooltip_message
+    message = "The branch could not be updated automatically because it has diverged from its upstream counterpart."
+    message << "<br>To discard the local changes and overwrite the branch with the upstream version, delete it here and choose 'Update Now' above." if can?(current_user, :push_code, @project)
+    message
+  end
+end