Add --conflict-style to standalone tools/diff/diff3.

* tools/diff/diff3.c
  (do_diff3): Add conflict style parameter.
  (main): Make argv const, add option parsing, add token map, clear error,
   pass conflict style, extend usage text.

* subversion/include/svn_diff.h
  (enum svn_diff_conflict_display_style_t): Add maintenance comment.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1591750 13f79535-47bb-0310-9956-ffa450edef68