We can't always rely on the network telling us when the download is
finished. Recognize it from the indexer itself.

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.

Currently only update_tips is used, but it prepares the way for
progress output during download.

Add a fetch refspec arguemnt and make the arguments (name, url,
refspec), as that order makes more sense.

Currently, git_remote_disconnect not only closes the connection but also
frees the underlying transport object, making it impossible to write
code like

	// fetch stuff
	git_remote_download()

	// close connection
	git_remote_disconnect()

	// call user provided callback for each ref
	git_remote_update_tips(remote, callback)

because remote->refs points to references owned by the transport object.
This means, we have an idling connection while running the callback for
each reference.

Instead, allow immediate disconnect and free the transport later in
git_remote_free().