ra_serf support for the r22294 RA cancellation callback.
ra_serf support for the r22294 RA cancellation callback.

* subversion/libsvn_ra_serf/util.c
  (svn_ra_serf__run_serf_context): Call the callback before blocking.

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