pipeline:diff fails when downstream app is paranoid

[bjeanes]

e.g.:

$ heroku pipelines:diff -a addons-staging
Fetching apps from pipeline... done
Fetching release info for all apps... done

addons-staging was not compared to addons as addons does not have any releases

$ heroku preauth -a addons
Two-Factor code: ********************************************

$ heroku pipelines:diff -a addons-staging
Fetching apps from pipeline... done
Fetching release info for all apps... done

addons-staging is ahead of addons by 10 commits:
  2973fb5  2015-12-07T23:50:03Z  Merge pull request #1319 from heroku/slug-validations (Bo Jeanes)
  79650fc  2015-12-07T20:44:38Z  Merge pull request #1312 from heroku/cdwort_small_addon_model_refactors (Bo Jeanes)
  8ec6b95  2015-12-07T20:28:42Z  Merge pull request #1316 from heroku/citext-slugs (Bo Jeanes)