Instad of each transport having its own function and logic to get to
its refs, store them directly in transport.

Leverage the new gitno_buffer to make the parsing and storing of the
refs use common code and get rid of the git_protocol struct.

This allows us to add capabilitites to both at the same time, keeps
them in sync and removes a lot of code.

gitno_buffer now uses a callback to fill its buffer, allowing us to
use the same interface for git and http (which uses callbacks).

For the transition, http is going to keep its own logic until the
git/common code catches up with the implied multi_ack that http
has. This also has the side-effect of making the code cleaner and more
correct regardingt he protocol.

Not everyone who indexes a packfile wants to put it in the standard
git repository location.

If it's not available, an error saying so will be returned when trying
to use a https:// URL.

This also unifies a lot of the network code to use git_transport in
many places instead of an socket descriptor.

These objects aren't considered as being advertised, so asking for
them will cause the remote end to close the connection. This makes the
checking in update_tips() unnecessary, because they don't get inserted
in the list.