Improve the interactive resolution of property conflicts.  Similar to
resolving a text conflict, show initially a single line such as

  local delete, incoming edit upon merge

and allow the user to display a 3-way diff with a 'dc - display conflict'
option.  Previously, for each property that is in conflict, we displayed the
entire contents of the property conflict reject file, which includes all of
the property conflicts on the node, and which is not very readable.

* subversion/svn/cl-conflicts.h,
  subversion/svn/cl-conflicts.c
  (svn_cl__get_human_readable_prop_conflict_description): New function.

* subversion/svn/conflict-callbacks.c
  (show_prop_conflict): New function.
  (prop_conflict_options): Add a 'dc - display conflict' option.
  (handle_prop_conflict): Work around a historical mix-up in the provided
    'theirs' and 'merged' and 'conflict-reject file' paths. Display a
    summary of the conflict instead of printing the '.prej' file. Show a
    3-way diff when the 'dc' option is selected.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1470246 13f79535-47bb-0310-9956-ffa450edef68