Fix progress reporting for ra_serf: serf reports incremental progress while 
svn_ra_progress_notify_func_t() needs total progress of bytes transferred.

* subversion/libsvn_ra_serf/ra_serf.h
  (svn_ra_serf__session_t): Add PROGRESS member to hold number of bytes 
   transferred so far. 

* subversion/libsvn_ra_serf/serf.c
  (svn_ra_serf__progress): Accumulate bytes transferred so far in session 
   and use for progress_func callback.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1516453 13f79535-47bb-0310-9956-ffa450edef68