When users call `git_http_client_read_body`, it should return 0 at the
end of a message.  When the `on_message_complete` callback is called,
this will set `client->state` to `DONE`.  In our read loop, we look for
this condition and exit.

Without this, when there is no data left except the end of message chunk
(`0\r\n`) in the http stream, we would block by reading the three bytes
off the stream but not making progress in any `on_body` callbacks.
Listening to the `on_message_complete` callback allows us to stop trying
to read from the socket when we've read the end of message chunk.