For easier FSFS f7 repair, put marker strings in front of the
index sections.  They are then easier to identify for a human.

* subversion/libsvn_fs_fs/index.c
  (L2P_STREAM_PREFIX,
   P2L_STREAM_PREFIX): Define the prefix strings that we will
                       put in front of the actual indexes.
  (STREAM_PREFIX_LEN): The corresponding buffer size.

  (packed_stream_open): Make the index stream generically check
                        the header prefix and skip it for the
                        actual stream parsing.
  (svn_fs_fs__l2p_index_append): Write the index header prefix.
  (auto_open_l2p_index): Update caller.
  (svn_fs_fs__p2l_index_append): Write the index header prefix.
  (auto_open_p2l_index): Update caller.

* subversion/libsvn_fs_fs/structure-indexes
  (L2P index on-disk format,
   P2L index on-disk format): Update format description.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1631567 13f79535-47bb-0310-9956-ffa450edef68