Simplify the wait handling in the transition based xml parser in lubsvn_ra_serf
by assuming that the xml parser handles well formedness.

* subversion/libsvn_ra_serf/xml.c
  (svn_ra_serf__xml_context_t): Just store the nesting level.
  (svn_ra_serf__xml_cb_start,
   svn_ra_serf__xml_cb_end,
   svn_ra_serf__xml_cb_cdata): Handle nesting as integer. Remove unneeded
     compares that the xml parser already performed.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1553908 13f79535-47bb-0310-9956-ffa450edef68