If a vendor repo is removed under one name and re-added under a
different name, *both* names would be listed and accepted as
vendor names.

For example:
  $ git vendor add mispeled http://some/url
  $ git vendor remove mispeled
  $ git vendor add correct http://some/url
  $ git vendor list
  correct:
  mispeled:

To fix, validate each vendor name by identifying the most recent
vendor name associated with the subtree directory.