Support specifying the parent for a cherry-pick in the REST API

When cherry-picking a merge, the REST API always selected the first
parent as reference. Sometimes users of the API want to choose a
specific parent just as they could on the command line using 'git
cherry-pick -m'. If the parent parameter isn't specified, it defaults
to 1.

Change-Id: I61dd0d9bee4a3c97354d52a18c66ae3567ab79fb

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/changes/CherryPickInput.java

@@ -17,4 +17,5 @@ package com.google.gerrit.extensions.api.changes;
 public class CherryPickInput {
   public String message;
   public String destination;
+  public Integer parent;
 }