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().
This allows the caller to update an internal structure or update the
user output with the tips that were updated.
While in the area, only try to update the ref if the value is
different from its old one.
