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