tuftool: bug when using path as URL

[Matthew James Briggs]

A bug was introduced in this commit 55a40cc

These lines do not work for tuftool gen rsa-key because the path to the key is for a file that does not yet exist. So we end up parsing the filepath with Url which fails like this:

    let input_as_path = Path::new(input);
    let url = if input_as_path.exists() {
        Url::from_file_path(input)
            .ok() // dump unhelpful `()` error
            .context(error::FileUrlSnafu {
                path: input_as_path.to_owned(),
            })?
    } else {
        Url::parse(input).context(error::UrlParseSnafu { url: input })?
    };

$ dir=/tmp/url-bug
rm -rf $dir
mkdir $dir

tuftool root init $dir/root.json
tuftool root gen-rsa-key \
   $dir/root.json \
   $dir/default.pem \
   --role root --role snapshot --role targets --role timestamp

Output:

Failed to parse URL "/tmp/url-bug/default.pem": relative URL without a base