We have the step-by-step method in the initialization function as we
want to remove references based on the list of references which are
already there, and we can use the convenience function for testing the
main push.
This function, similar in style to git_remote_fetch(), performs all the
steps required for a push, with a similar interface.
The remote callbacks struct has learnt about the push callbacks, letting
us set the callbacks a single time instead of setting some in the remote
and some in the push operation.
If the certificate validation fails (or always in the case of ssh),
let the user decide whether to allow the connection.
The data structure passed to the user is the native certificate
information from the underlying implementation, namely OpenSSL or
WinHTTP.
