Block off commands on a server for certain user groups.

This feature adds two new options to gerrit.config file: upload and
receive with the allowGroup attribute, that restrict to some specific
groups the ability to run upload/receive commands on the server.

[sp: All bugs are mine, I refactored the code a bit from the original]

Change-Id: Ibd31bd11234e429f8b0201bbb03099f737281f21
Signed-off-by: Shawn O. Pearce <sop@google.com>

gerrit-sshd/src/main/java/com/google/gerrit/sshd/commands/Receive.java

@@ -58,6 +58,10 @@ final class Receive extends AbstractGitCommand {
 
   @Override
   protected void runImpl() throws IOException, Failure {
+    if (!projectControl.canRunReceivePack()) {
+      throw new Failure(1, "fatal: receive-pack not permitted on this server");
+    }
+
     final ReceiveCommits receive = factory.create(projectControl, repo);
 
     ReceiveCommits.Capable r = receive.canUpload();