ra_serf: Implement ability to override automatic bulk-mode selection using 
tristate bulk-updates option.

* subversion/libsvn_ra_serf/ra_serf.h
  (struct svn_ra_serf__session_t): Change type of bulk_updates memebr to 
   svn_tristate_t.
* subversion/libsvn_ra_serf/serf.c
  (load_config): Get 'http-bulk-updates' runtime option as tristate with 
   'yes'/'no'/'auto' values.
* subversion/libsvn_ra_serf/update.c
  (make_update_reporter): Override bulk-mode selection logic if user 
   explicitly specified desired update editor mode.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1436351 13f79535-47bb-0310-9956-ffa450edef68