In preparation of having a single file containing rev data and indexes,
teach our index data stream to use only a sub-section of a given file.
However, don't expose that ability, yet.

* subversion/libsvn_fs_fs/index.c
  (svn_fs_fs__packed_number_stream_t): Add members to select the offset
                                       range that the stream covers. 
  (packed_stream_read): Limit to the upper end of the stream.
  (packed_stream_open): Set the range for the stream data to the full
                        index file, for now.
  (packed_stream_seek,
   packed_stream_offset): Use / report stream offsets relative to the
                          beginning of the stream instead of the under-
                          lying file.  That will ensure identical content
                          for the indexes before and after combining
                          them with the data file.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1604416 13f79535-47bb-0310-9956-ffa450edef68