Add CURRENT_ACTIONS to revision information

This exports any plugin actions that apply to the current revision,
telling the UI what can be invoked over the REST API.

Change-Id: Ieb5e182d43628497c20077b3bc746a6b5bf71abb

gerrit-common/src/main/java/com/google/gerrit/common/changes/ListChangesOption.java

@@ -37,7 +37,10 @@ public enum ListChangesOption {
   DETAILED_ACCOUNTS(7),
 
   /** Include messages associated with the change. */
-  MESSAGES(9);
+  MESSAGES(9),
+
+  /** Include allowed actions client could perform. */
+  CURRENT_ACTIONS(10);
 
   private final int value;