Add support for relative incremental dumps.
Add support for relative incremental dumps.

* tools/server-side/svn-backup-dumps.py:
  (): Update documentation.
  (SvnBackup.__init__): Set new option self.__relative_incremental.
  (SvnBackup.get_last_dumped_rev): New function.
  (SvnBackup.export_relative_incremental): New function.
  (SvnBackup.execute): Call export_relative_incremental if requested.
  ("__main__"): Add option "-i" to OptionParser.

Patch by: Mark Stead <mark@stead.id.au>

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion