Fix issue 3260: problems with progress callback.

* subversion/include/svn_client.h
  (svn_client_ctx_t): Add PROGRESS member.

* subversion/libsvn_client/ra.c
  (callback_baton_t): Add LAST_PROGRESS member.
  (progress_func): New. Accumulate progress information for all sessiona 
   and invoke real callback with cumulative progress.
  (svn_client__open_ra_session_internal): Use progress_func.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1516511 13f79535-47bb-0310-9956-ffa450edef68