Allow ra_svn protocol parsing to use svn_tristate_t instead of
a non-robust combination of TRUE, FALSE, and a custom macro.

* subversion/include/private/svn_ra_svn_private.h
  (svn_ra_svn__parse_tuple): Add '3' format code, like 'B'.
    Deprecate 'B'.

* subversion/libsvn_ra_svn/marshal.c
  (svn_ra_svn__parse_tuple): Ditto.

* subversion/svnserve/serve.c
  (update, switch_cmd): Use '3' (svn_tristate_t) instead of 'B'.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1476641 13f79535-47bb-0310-9956-ffa450edef68