Got tired of all the -Wtautological-constant-out-of-range-compare warnings.

* subversion/include/svn_wc.h (svn_wc_conflict_choice_t):
   Add constant svn_wc_conflict_choose_undefined, and make sure that
   the existing enumeration values remain unchanged.
* subversion/svn/conflict-callbacks.c:
   Use this constant instead of untyped -1 for conflict
   resolution choices. Silences a number of warnings with clang on OSX.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1588665 13f79535-47bb-0310-9956-ffa450edef68