Fix the svn_fs_get_file_delta_stream for FSFS and FSX.  We may only
use the on-disk delta representation, if that is actually a delta
between source and target node.  A self-delta will not do.

* subversion/libsvn_fs_fs/cached_data.c
  (svn_fs_fs__get_file_delta_stream): Don't accept self-delta reps
                                      as a short-cut.

* subversion/libsvn_fs_x/cached_data.c
  (svn_fs_x__get_file_delta_stream): Ditto.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1554978 13f79535-47bb-0310-9956-ffa450edef68