If a merged property value exists, make the 'dc' conflict prompt option use
the merged value as 'mine' for display. Else, users might get confused if
they edit a property, run 'dc', and see the value from before the edit.

* subversion/svn/conflict-callbacks.c
  (show_prop_conflict, merge_prop_conflict): Add merged_abspath parameter.
   Use it as 'my' version if non-NULL.
  (edit_prop_conflict): For now, pass NULL for the merged file to
   merge_prop_conflict(), so that repeated edits always start with
   the same original values. This behaviour can be reconsidered later.
  (handle_prop_conflict): Pass the path to the merged file, which might be
   NULL, to show_prop_conflict().


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1480669 13f79535-47bb-0310-9956-ffa450edef68