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).

Recognize and report server-side error messages

When e.g. a repository isn't found, the server sends an error saying
so. Put that error message in our error buffer.

Signed-off-by: schu <schu-github@schulog.org>