Running unzip is relatively expensive.  Therefore skip the decoding
of txdelta windows in block_read() if we already have them in cache.

* subversion/include/private/svn_delta_private.h
  (svn_txdelta__read_svndiff_window_sizes): Declare new utility API
   that evaluates just enough information to let us skip the window.

* subversion/libsvn_delta/svndiff.c
  (svn_txdelta__read_svndiff_window_sizes): Implement.

* subversion/libsvn_fs_fs/cached_data.c
  (cache_windows): Don't decode windows that we already have in cache.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1555284 13f79535-47bb-0310-9956-ffa450edef68