Extend the get_file_delta_stream optimization to self-deltas.

Dump code will request self-deltas (delta against a NULL source)
for newly added files.  FSFS will often use the same representation
on disk.

* subversion/libsvn_fs_fs/cached_data.c
  (get_storaged_delta_stream): New utility function factored out from ...
  (svn_fs_fs__get_file_delta_stream): ... this.  Add another special case
                                      if a self-delta has been requested.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1555297 13f79535-47bb-0310-9956-ffa450edef68