Expose the --allow-mixed-revisions merge option in JavaHL.

[in subversion/bindings/javahl]

* src/org/apache/subversion/javahl/ISVNClient.java
  (ISVNClent.merge): Add overloads that accept an allowMixedRev parameter.
   Update documentation for previous overloads.
* src/org/apache/subversion/javahl/SVNClient.java
  (ISVNClent.merge): Update native prototypes with the allowMixedRev parameter.
   Add wrapper implementations for the previous versions.

* native/SVNClient.h (SVNClient::merge): Add allowMixedRev parameters.
* native/SVNClient.cpp (SVNClient::merge): Likewise, and pass them on
   to the core API.
* native/org_apache_subversion_javahl_SVNClient.cpp:
   Update the callers in the JNI wrapper implementations.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1567602 13f79535-47bb-0310-9956-ffa450edef68