the pointer where the creds are to be stored
a smart transport
the user we saw on the url (if any)
available methods for authentication
the return value of the callback: 0 for no error, git_error_code.GIT_PASSTHROUGH to indicate that there is no callback registered (or the callback refused to provide credentials and callers should behave as if no callback was set), or < 0 for an error
Call the credentials callback for this transport