Make it easier to use tools/diff/diff3 in a --diff3-cmd script by
accepting all the options passed when a diff3 program is invoked.
Use '-L' to label the output, simply ignore '-E' and '-m'.  This
allows a --diff3-cmd script to invoke diff3 as:

    diff3 --conflict-style modified-latest $@

* tools/diff/diff3.c
  (do_diff3): Add conflict label parameters.
  (main): Add '-L' and accept up to 3 instances, add '-m' and '-E' and
   ignore, tweak usage text.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1591871 13f79535-47bb-0310-9956-ffa450edef68