Implement a file-based diff utility in JavaHL.

[in subversion/bindings/javahl/src/org/apache/subversion/javahl]
* SVNUtil.java (SVNUtil.FileDiff): New static method.
  (SVNUtil.FileMerge): Fix docstring.
* util/DiffLib.java (SVNUtil.FileDiff): Implement.
  (SVNUtil.nativeFileDiff): New native wrapper declaration.
  (SVNUtil.FileMerge): Tweak implementation.

[in subversion/bindings/javahl/native]
* org_apache_subversion_javahl_util_DiffLib.cpp
  (Java_org_apache_subversion_javahl_util_DiffLib_nativeFileDiff):
   Native implementation of SVNUtil.FileDiff.

[in subversion/bindings/javahl/tests/org/apache/subversion/javahl]
* UtilTests.java (UtilTests.fileContentsPrefix): Add terminating newline.
  (UtilTests.testFileDiff): New test case.
  (UtilTests.testFileMerge): Add newlines to file contents and adjust
   the expected result.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1512490 13f79535-47bb-0310-9956-ffa450edef68