git-cvsserver: handle CVS 'noop' command.

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>