The cancellation editor has been discarding the wrapped editor's

The cancellation editor has been discarding the wrapped editor's
abort_edit callback, and all cancellation-wrapped editors have
been using the default editor's no-op abort_edit.

* subversion/include/svn_delta.h
  (svn_delta_editor_t): Add comment about updating cancellation and
    default editors when adding new callbacks.

* subversion/libsvn_delta/cancel.c
  (abort_edit): Add new wrapper.
  (svn_delta_get_cancellation_editor): Set abort_edit.

git-svn-id: http://svn.collab.net/repos/svn/trunk@30246 612f8ebc-c883-4be0-9ee0-a4e9ef946e3a