Add a custom transport definition, to be used in addition to the built-in set of transports that come with libgit2.
The caller is responsible for synchronizing calls to git_transport_register and git_transport_unregister with other calls to the library that instantiate transports.
The scheme (ending in "://") to match, i.e. "git://"
A fixed parameter to pass to cb at creation time
0 or an error code