Review requests with a repository but no published diffs made rbt info

request diff revision 0, which failed with an uncaught API error. This

now skips the diff section in that case, and reports a clear error when

a requested diff revision does not exist.

Ran rbt info on an attachment-only review request with no diffs.
Ran rbt info on a review request with diffs and passed in the

  optional diff revision param with a revision that didn't exist.