git-cvsserver: handle CVS 'noop' command.

The CVS protocol documentation, found at http://www.wandisco.com/techpubs/cvs-protocol.pdf states the following about the 'noop' command: Response expected: yes. This request is a null command in the sense that it doesn't do anything, but merely (as with any other requests expecting a response) sends back any responses pertaining to pending errors, pending Notified responses, etc. In accordance with this, the correct way to handle the 'noop' command, when issued by a client, is to call req_EMPTY. The 'noop' command is called by some CVS clients, notably TortoiseCVS, thus making it desirable for git-cvsserver to respond to the command rather than choking on it as unknown. Signed-off-by: Stefan Karpinski <stefan.karpinski@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>