old_contents: GITHUB_WEBHOOK_SECRET="nothing to see here"
  GITHUB_OAUTH_CLIENT_ID="nothing to see here"
  GITHUB_OAUTH_CLIENT_SECRET="nothing to see here"
  GITHUB_STATUS_OAUTH_TOKEN="nothing to see here"
  COVERALLS_REPO_TOKEN="nothing to see here"
  CODECOV_REPO_TOKEN="nothing to see here"
  FREEBSDCI_OAUTH_TOKEN="nothing to see here"
  FQDN="buildog.julialang.org"
  BUILDBOT_BRANCH="master"
  db_user="nothing to see here"
  db_password="nothing to see here"
  DOCUMENTER_KEY="nothing to see here"
  
new_contents: GITHUB_WEBHOOK_SECRET="nothing to see here"
  GITHUB_OAUTH_CLIENT_ID="nothing to see here"
  GITHUB_OAUTH_CLIENT_SECRET="nothing to see here"
  GITHUB_STATUS_OAUTH_TOKEN="nothing to see here"
  COVERALLS_REPO_TOKEN="nothing to see here"
  CODECOV_REPO_TOKEN="nothing to see here"
  FREEBSDCI_OAUTH_TOKEN="nothing to see here"
  FQDN="buildog.julialang.org"
  BUILDBOT_BRANCH="master"
  db_user="nothing to see here"
  db_password="nothing to see here"
  DOCUMENTER_KEY="nothing to see here"
  MACOS_CODESIGN_IDENTITY="nothing to see here"
  
subject: Add mock value for `MACOS_CODESIGN_IDENTITY`