Add notify section in project.config

The notify section allows project owners to include emails to users
directly from project.config. This removes the need to create fake
user accounts to always BCC a group mailing list. For example:

  [notify "dev-group"]
    email = dev-team <dev-team@example.com>
    filter = branch:master visibleto:dev

Internal groups may also be used as a mailing list, automatically
BCCing any user that is a member of the group if the group can see
the change:

  [access "refs/heads/*"]
    read = group Developers
  [notify "reviewers"]
    email = group Developers

Change-Id: I02bd05b562e420a4742ff27ffacad8f20648e4f0

gerrit-server/src/main/java/com/google/gerrit/server/query/Predicate.java

@@ -43,6 +43,12 @@ import java.util.List;
  * @type <T> type of object the predicate can evaluate in memory.
  */
 public abstract class Predicate<T> {
+  /** A predicate that matches any input, always, with no cost. */
+  @SuppressWarnings("unchecked")
+  public static <T> Predicate<T> any() {
+    return (Predicate<T>) Any.INSTANCE;
+  }
+
   /** Combine the passed predicates into a single AND node. */
   public static <T> Predicate<T> and(final Predicate<T>... that) {
     if (that.length == 1) {
@@ -120,4 +126,36 @@ public abstract class Predicate<T> {
 
   @Override
   public abstract boolean equals(Object other);
+
+  private static class Any<T> extends Predicate<T> {
+    private static final Any<Object> INSTANCE = new Any<Object>();
+
+    private Any() {
+    }
+
+    @Override
+    public Predicate<T> copy(Collection<? extends Predicate<T>> children) {
+      return this;
+    }
+
+    @Override
+    public boolean match(T object) {
+      return true;
+    }
+
+    @Override
+    public int getCost() {
+      return 0;
+    }
+
+    @Override
+    public int hashCode() {
+      return System.identityHashCode(this);
+    }
+
+    @Override
+    public boolean equals(Object other) {
+      return other == this;
+    }
+  }
 }