Fix deprecation warnings in BDB FS code.

* subversion/libsvn_fs_base/reps-strings.c
  (svn_fs_base__rep_deltify): use the new svn_txdelta_to_svndiff3 and
   svn_txdelta2 instead of their predecessors

* subversion/libsvn_fs_base/tree.c
  (base_get_file_delta_stream): use the new svn_txdelta2


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1446864 13f79535-47bb-0310-9956-ffa450edef68