Display command description in ssh gerrit --help

Show one line description for each ssh command. Make COMMAND
argument optional, so that --help argument is not needed.

This approach is working for core gerrit commands, aliases, nested
commands and plugin's own ssh commands.

To provide a description ssh command must be annotated with
CommandMetaData annotation, i. e.:

@CommandMetaData(name="print", descr="Print greeting in different languages")
public final class PrintHelloWorldCommand extends SshCommand { ...

Syntactic sugar for command registration is provided to reflect the fact,
that both name and description are included in CommandMetaData annotation.

To register command and alias in plugin:
protected void configureCommands() {
  command(PrintHelloWorldCommand.class);
  alias("say-hello", PrintHelloWorldCommand.class);
[...]

With the outcome:
$ ssh gerrit helloworld
Available commands of helloworld are:

   print       Print greeting in different languages
   say-hello   Print greeting in different languages

Change-Id: I2e5440378023ecc5425092d8131f121da2f20a30

gerrit-sshd/src/main/java/com/google/gerrit/sshd/CommandProvider.java

@@ -0,0 +1,38 @@
+// Copyright (C) 2013 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.sshd;
+
+import com.google.inject.Provider;
+
+import org.apache.sshd.server.Command;
+
+final class CommandProvider {
+
+  private final Provider<Command> provider;
+  private final String description;
+
+  CommandProvider(final Provider<Command> p, final String d) {
+    this.provider = p;
+    this.description = d;
+  }
+
+  public Provider<Command> getProvider() {
+    return provider;
+  }
+
+  public String getDescription() {
+    return description;
+  }
+}