For FSFS format 7, use code to construct the index part of r0
instead of a hard-coded constant.

* subversion/libsvn_fs_fs/fs_fs.c
  (write_revision_zero): Add SCRATCH_POOL parameter since there are plenty
                         of temporary objects.  Construct the item list /
                         index entries, use the index utilities to turn
                         them into the two 2 indexes and add them to r0.
  (svn_fs_fs__create): Update caller.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1606554 13f79535-47bb-0310-9956-ffa450edef68