On the fsx-id branch:  Introduce ID utility functions that create / init
transaction and revision root noderev IDs.  Use them where appropriate.

* subversion/libsvn_fs_x/id.h
  (svn_fs_x__init_txn_root,
   svn_fs_x__init_rev_root): Declare the new functions.

* subversion/libsvn_fs_x/id.c
  (svn_fs_x__init_txn_root,
   svn_fs_x__init_rev_root): Implement them.

* subversion/libsvn_fs_x/cached_data.c
  (svn_fs_x__rev_get_root):  Call on of the new functions.

* subversion/libsvn_fs_x/transaction.c
  (create_new_txn_noderev_from_rev): Same.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsx-id@1647756 13f79535-47bb-0310-9956-ffa450edef68