Project index: Add parent predicate

Enabling the search by parent enables the rewrite
of the ListChildProjects REST-API that extracts the children
of a project. Using the Lucene index avoids the full scan
of the ProjectCache and thus speedup the extraction and
reduce the JVM heap consumption.

Bug: Issue 10401
Change-Id: I568d6ba0e5a0bff391f57e2357de6d059d77fe5d

java/com/google/gerrit/server/query/project/ProjectPredicates.java

@@ -27,6 +27,10 @@ public class ProjectPredicates {
     return new ProjectPredicate(ProjectField.NAME, nameKey.get());
   }
 
+  public static Predicate<ProjectData> parent(Project.NameKey parentNameKey) {
+    return new ProjectPredicate(ProjectField.PARENT_NAME, parentNameKey.get());
+  }
+
   public static Predicate<ProjectData> inname(String name) {
     return new ProjectPredicate(ProjectField.NAME_PART, name.toLowerCase(Locale.US));
   }

java/com/google/gerrit/server/query/project/ProjectQueryBuilder.java

@@ -44,6 +44,11 @@ public class ProjectQueryBuilder extends QueryBuilder<ProjectData> {
     return ProjectPredicates.name(new Project.NameKey(name));
   }
 
+  @Operator
+  public Predicate<ProjectData> parent(String parentName) {
+    return ProjectPredicates.parent(new Project.NameKey(parentName));
+  }
+
   @Operator
   public Predicate<ProjectData> inname(String namePart) {
     if (namePart.isEmpty()) {