On the fsfs-format7 branch:  Simplify svn_fs_fs__rev_get_root for format7
repositories because their ID is fixed for every revision and doesn't
require a lookup anymore.

* subversion/libsvn_fs_fs/id.h
  (svn_fs_fs__id_create_root): declare new API

* subversion/libsvn_fs_fs/id.c
  (svn_fs_fs__id_create_root): implement it

* subversion/libsvn_fs_fs/cached_data.c
  (svn_fs_fs__rev_get_root): directly construct and return the root ID
  (block_read_noderev): no need to cache root IDs anymore



git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1469500 13f79535-47bb-0310-9956-ffa450edef68