- rephrase the documentation.
- remove unnecessary #:ensure #f key in call to (cache-directory).
- clarify what happens when string->uri fails and yields a warning by
  passing a 'bad-repo symbol and commenting at its elimination site.
- make a separate function of get-uri now that it's become as large as
  its caller repo-type.
- mention the possibility to call --repo several times in help message.
- add points at the end of sentences in commit message and in this
  e-mail.