PatchSet: Add @Deprecated annotation to deprecated method

This exposes the deprecation to tools that do not read the javadoc,
and fixes an Error Prone problem[1].

[1] http://errorprone.info/bugpattern/DepAnn

Change-Id: I46effb8bb9a633705640b2629a0df6c4aa81df91

gerrit-reviewdb/src/main/java/com/google/gerrit/reviewdb/client/PatchSet.java

@@ -30,10 +30,12 @@ public final class PatchSet {
     return Id.fromRef(name) != null;
   }
 
-  /** Is the reference name a change reference?
+  /**
+   * Is the reference name a change reference?
    *
    * @deprecated use isChangeRef instead.
    **/
+  @Deprecated
   public static boolean isRef(String name) {
     return isChangeRef(name);
   }