Annotate RefUpdate and CommitReceived as Singleton

These should be singletons so that they are not instantiated every
time a hook is executed.

Change-Id: I32e67a85c24138148c1cff97d76144030942930e

src/main/java/com/googlesource/gerrit/plugins/hooks/CommitReceived.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/hooks/CommitReceived.java b/src/main/java/com/googlesource/gerrit/plugins/hooks/CommitReceived.java
index 02800fc..e08a043 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/hooks/CommitReceived.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/hooks/CommitReceived.java
@@ -20,10 +20,12 @@
 import com.google.gerrit.server.git.validators.CommitValidationListener;
 import com.google.gerrit.server.git.validators.CommitValidationMessage;
 import com.google.inject.Inject;
+import com.google.inject.Singleton;
 import java.util.Collections;
 import java.util.List;
 import org.eclipse.jgit.lib.ObjectId;
 
+@Singleton
 public class CommitReceived implements CommitValidationListener {
   private final SynchronousHook hook;
   private final HookFactory hookFactory;

src/main/java/com/googlesource/gerrit/plugins/hooks/RefUpdate.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/hooks/RefUpdate.java b/src/main/java/com/googlesource/gerrit/plugins/hooks/RefUpdate.java
index d9b38e4..2182614 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/hooks/RefUpdate.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/hooks/RefUpdate.java
@@ -21,10 +21,12 @@
 import com.google.gerrit.server.git.validators.ValidationMessage;
 import com.google.gerrit.server.validators.ValidationException;
 import com.google.inject.Inject;
+import com.google.inject.Singleton;
 import java.util.Collections;
 import java.util.List;
 import org.eclipse.jgit.lib.ObjectId;
 
+@Singleton
 public class RefUpdate implements RefOperationValidationListener {
   private final SynchronousHook hook;
   private final HookFactory hookFactory;