Expose a utility to resolve relative extrnalls URLs in JavaHL.

[in subversion/bindings/javahl]

* src/org/apache/subversion/javahl/SVNUtil.java
  (SVNUtil.resolveExternalsUrl): New static method.
* src/org/apache/subversion/javahl/util/PropLib.java
  (PropLib.resolveExternalsUrl): New native method.
  (PropLib.canonicalizeSvnProperty, PropLib.unparseExternals):
   Fix references in docstrings.

* native/ExternalItem.hpp, native/ExternalItem.cpp
  (ExternalItem::get_external_item): New public method.
* native/org_apache_subversion_javahl_util_PropLib.cpp
  (Java_org_apache_subversion_javahl_util_PropLib_resolveExternalsUrl):
   New; implementation of PropLib.resolveExternalsUrl.

* tests/org/apache/subversion/javahl/UtilTests.java
  (UtilTests.testResolveExternalsUrl): New testcase.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1540921 13f79535-47bb-0310-9956-ffa450edef68