Fix a Java 8 compiler warning in JavaHL.

* subversion/bindings/javahl/src/org/apache/subversion/javahl/types/ExternalItem.java
  (ExternalItem.hashCode): Override this method here since equals() was also
   overriden.
  (ExternalItem.serialVersionUID): Update comment.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1605871 13f79535-47bb-0310-9956-ffa450edef68