git: ignore http auth for ssh url

This change ensures that http-basic auth credentials are only passed to
dulwich when the remote url uses http/https schemes.

In addition to the above, it is now ensured that username/password
parameters are not passed through to dulwich unless both username and
password are configured explicitly. This is to ensure that dulwich does
not bail out if it detects a username in the url
(eg: `ssh://git@github.com`).